Guide

EPUB E-Book Format: The Complete Technical Guide

Q: Why is EPUB essentially a ZIP file, and what can I do with that?

EPUB is literally a renamed ZIP file — rename any .epub to .zip and unzip it to see all the XHTML, CSS, images, and metadata files inside. This design was intentional: ZIP provides compression, random access (extract individual files without reading the whole archive), and a well-understood container format. Practical implications: you can inspect or edit EPUB contents with any ZIP tool or text editor, extract images with unzip, debug layout issues by opening the XHTML files in a browser, bulk-edit metadata by modifying content.opf directly, and create EPUBs programmatically by assembling the files and zipping them (with mimetype stored first, uncompressed).

Q: Why is EPUB essentially a ZIP file, e what can I do com that?

EPUB is literally a renamed ZIP arquivo — rename any .epub to .zip e unzip it to see all the XHTML, CSS, images, e metadata arquivos inside. This design was intentional: ZIP fornece compressão, random access (extract individual arquivos sem reading the whole archive), e a well-understood container formato. Practical implications: you can inspect ou editar EPUB contents com any ZIP tool ou text editor, extract images com unzip, debug layout issues by abrindo the XHTML arquivos in um navegador, bulk-edit metadata by modifying content.opf directly, e create EPUBs programmatically by assembling os arquivos e zipping them (with mimetype stored first, não comprimido).

Q: Why is EPUB essentially a ZIP file, und what can I do mit that?

EPUB is literally a renamed ZIP Datei — rename any .epub to .zip und unzip it to see all the XHTML, CSS, images, und metadata Dateien inside. This design was intentional: ZIP bietet Komprimierung, random access (extract individual Dateien ohne reading the whole archive), und a well-understood Container Format. Practical implications: you can inspect oder bearbeiten EPUB contents mit any ZIP tool oder text editor, extract images mit unzip, debug layout issues by opening the XHTML Dateien in ein Browser, bulk-edit metadata by modifying content.opf directly, und create EPUBs programmatically by assembling die Dateis und zipping them (with mimetype stored first, uncompressed).

PC By Pablo Cirre • Updated May 7, 2026

What Is EPUB?

EPUB — Electronic Publication — is the open standard for digital books published by the W3C EPUB Working Group (formerly the IDPF). First released in 2007 as EPUB 2.0.1, it replaced the older Open eBook (OEB) format. The current version is EPUB 3.3 (2023), which aligns the format with modern web technologies: HTML5, CSS3, SVG, JavaScript, and MathML.

EPUB is used by virtually every digital publishing ecosystem except Amazon's proprietary AZW/MOBI/KFX formats. Major devices and platforms include: Apple Books, Kobo, Barnes & Noble Nook, Google Play Books, Calibre, FBReader, Adobe Digital Editions, and most open-source e-readers.

Physical Structure: EPUB is a ZIP

Every EPUB file is a renamed ZIP archive. Unzipping any .epub file reveals:

mimetype                          ← must be first file, uncompressed
META-INF/
  container.xml                   ← points to the OPF file
  encryption.xml (optional)
  signatures.xml (optional)
OEBPS/ (or any name)
  content.opf                     ← package document (spine, manifest, metadata)
  toc.ncx                         ← legacy EPUB 2 navigation
  nav.xhtml                       ← EPUB 3 navigation document
  Text/
    chapter01.xhtml
    chapter02.xhtml
    ...
  Images/
    cover.jpg
    figure01.png
    ...
  Styles/
    stylesheet.css
  Fonts/
    MyFont.otf

The mimetype file contains exactly application/epub+zip — no newline, no BOM — and must be the first entry in the ZIP, stored uncompressed. This allows MIME-type sniffing without full decompression.

Package Document (OPF)

The OPF (Open Packaging Format) file is the manifest of the publication:

<?xml version="1.0" encoding="UTF-8"?>
<package xmlns="http://www.idpf.org/2007/opf" version="3.0"
         xml:lang="en" unique-identifier="bookid">

  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
    <dc:identifier id="bookid">urn:isbn:9780000000000</dc:identifier>
    <dc:title>The Complete EPUB Guide</dc:title>
    <dc:creator id="author">Jane Doe</dc:creator>
    <dc:language>en</dc:language>
    <dc:publisher>Tech Books Inc.</dc:publisher>
    <dc:date>2024-01-15</dc:date>
    <meta property="dcterms:modified">2024-01-15T10:00:00Z</meta>
    <meta name="cover" content="cover-image"/>
  </metadata>

  <manifest>
    <item id="nav" href="nav.xhtml" media-type="application/xhtml+xml"
          properties="nav"/>
    <item id="cover-image" href="Images/cover.jpg" media-type="image/jpeg"
          properties="cover-image"/>
    <item id="ch01" href="Text/chapter01.xhtml"
          media-type="application/xhtml+xml"/>
    <item id="ch02" href="Text/chapter02.xhtml"
          media-type="application/xhtml+xml"/>
    <item id="css" href="Styles/stylesheet.css" media-type="text/css"/>
  </manifest>

  <spine>
    <itemref idref="ch01"/>
    <itemref idref="ch02"/>
  </spine>

</package>

Key components:

<metadata>: Dublin Core elements (title, creator, language, identifier) plus EPUB-specific meta properties.
<manifest>: lists every file in the publication with its media type and optional properties.
<spine>: defines the reading order — which documents the e-reader presents in sequence.
<guide> (EPUB 2): legacy navigation landmarks (now replaced by <landmarks> in nav.xhtml).

Navigation Document (nav.xhtml)

EPUB 3's navigation document replaces the EPUB 2 NCX file:

<?xml version="1.0" encoding="UTF-8"?>
<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:epub="http://www.idpf.org/2007/ops">
<head><title>Table of Contents</title></head>
<body>
  <nav epub:type="toc" id="toc">
    <h1>Contents</h1>
    <ol>
      <li><a href="Text/chapter01.xhtml">Chapter 1: Introduction</a></li>
      <li><a href="Text/chapter02.xhtml#section2">Chapter 2: Deep Dive</a>
        <ol>
          <li><a href="Text/chapter02.xhtml#sub1">2.1 Sub-topic</a></li>
        </ol>
      </li>
    </ol>
  </nav>

  <nav epub:type="landmarks">
    <ol>
      <li><a epub:type="toc" href="nav.xhtml#toc">Table of Contents</a></li>
      <li><a epub:type="bodymatter" href="Text/chapter01.xhtml">Start</a></li>
    </ol>
  </nav>
</body>
</html>

EPUB 2 vs. EPUB 3

Feature	EPUB 2.0.1	EPUB 3.3
Content documents	XHTML 1.1	HTML5 / XHTML5
CSS support	CSS 2.1	CSS3
SVG	Limited	Full SVG 1.1
JavaScript	Not supported	Supported (scripted)
MathML	Not supported	Native
Audio/Video	Not supported	HTML5 `<audio>` / `<video>`
Navigation	NCX (toc.ncx)	nav.xhtml
Fonts	OpenType	OpenType, WOFF, WOFF2
Accessibility	Limited	DPUB-ARIA, Schema.org
Fixed layout	Limited	Full specification
Pronunciation	Not supported	PLS + SSML

EPUB 3 is the standard for new publications. EPUB 2 is still read by all major readers for backwards compatibility.

Fixed Layout vs. Reflowable

Reflowable EPUB (default): text reflows to fit the reader's screen, font size, and orientation. Ideal for novels, textbooks, and most prose.

Fixed layout EPUB: each page has a fixed pixel size, like a PDF. Used for children's picture books, comics, and design-heavy magazines:

<!-- OPF metadata for fixed layout -->
<meta property="rendition:layout">pre-paginated</meta>
<meta property="rendition:orientation">landscape</meta>
<meta property="rendition:spread">landscape</meta>

<!-- Per-item override in manifest -->
<item id="page01" href="page01.xhtml" media-type="application/xhtml+xml"
      properties="rendition:layout-pre-paginated rendition:spread-none"/>

/* In the content document */
html { width: 1024px; height: 768px; }

Media Overlays (Read-Aloud)

EPUB 3 supports synchronized text and audio via SMIL Media Overlays — the e-reader highlights text as it reads aloud:

<smil xmlns="http://www.w3.org/ns/SMIL" version="3.0">
  <body>
    <seq id="s1" epub:textref="Text/chapter01.xhtml"
         epub:type="bodymatter chapter">
      <par id="par01">
        <text src="Text/chapter01.xhtml#para01"/>
        <audio src="Audio/chapter01.mp3" clipBegin="0:00:00.000"
               clipEnd="0:00:04.500"/>
      </par>
    </seq>
  </body>
</smil>

This is the basis for accessible read-aloud books for visually impaired readers.

DRM and Adobe Digital Editions

DRM (Digital Rights Management) in EPUB is typically implemented via Adobe Content Server (for ACS4/ADEPT DRM) or Apple's FairPlay. DRM restrictions are stored in META-INF/encryption.xml. DRM-protected EPUBs cannot be read by all readers — only those licensed by the DRM provider.

Kindle note: Amazon uses its own formats (AZW, MOBI, KFX) that are not EPUB. Conversion from EPUB to Kindle formats is handled by Calibre or Amazon's online converter. Kindle Scribe can now open EPUB files natively (from 2023).

Creating EPUB Files

Pandoc (command-line, free):

pandoc input.docx -o output.epub --epub-cover-image=cover.jpg
pandoc input.md -o book.epub --toc --metadata title="My Book"

Sigil (free, open-source EPUB editor): full GUI for creating and editing EPUB 2/3 with syntax highlighting and validation.

Calibre (free): converts between formats and can edit EPUB metadata. Not recommended for creation of polished EPUBs.

Adobe InDesign: industry standard for publishing — exports directly to EPUB 3 with fixed or reflowable layout.

Jutoh / Vellum / Atticus: consumer tools optimized for self-publishing authors.

Validating EPUB

EPUBCheck (official W3C validator):

java -jar epubcheck.jar book.epub
java -jar epubcheck.jar book.epub --json report.json

EPUBCheck validates:

ZIP structure (mimetype placement, required files)
OPF wellformedness and required metadata
Navigation document structure
All referenced files exist in manifest
HTML/CSS validity
Accessibility metadata (EPUB 3)

A valid EPUB passes EPUBCheck with 0 errors and 0 warnings.

Converting EPUB

EPUB → PDF: Calibre ebook-convert input.epub output.pdf, or Adobe InDesign export.
EPUB → MOBI/AZW3: Calibre, or Amazon KindleGen (discontinued; now Amazon's online converter).
EPUB → DOCX: Pandoc pandoc input.epub -o output.docx.
EPUB → HTML: unzip book.epub -d book_html/ — the XHTML files are directly readable.
DOCX → EPUB: Pandoc, or Word → Save as Filtered HTML → Sigil.
PDF → EPUB: Calibre (lossy — layout may degrade), or professional OCR workflow.

Best Practices

Validate with EPUBCheck before distributing — invalid EPUBs break on many readers.
Use UTF-8 encoding in all content documents and declare it in the XML declaration.
Include cover image in the manifest with properties="cover-image" for store thumbnails.
Embed only required fonts — unused font files increase file size unnecessarily.
Test on multiple readers: Kindle Previewer, Apple Books, Kobo, FBReader, and browsers via epub.js.
Separate content from presentation — keep CSS in external stylesheets, not inline styles.
Use semantic markup (epub:type attributes) for accessibility and navigation.
Keep image file sizes small: JPEG for photos (< 200 KB), PNG for diagrams with few colors.
Preserve NCX file (toc.ncx) even in EPUB 3 for compatibility with EPUB 2 readers.
Use Pandoc for batch conversions — it handles most office formats to EPUB reliably.

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Frequently Asked Questions

EPUB is literally a renamed ZIP file — rename any .epub to .zip and unzip it to see all the XHTML, CSS, images, and metadata files inside. This design was intentional: ZIP provides compression, random access (extract individual files without reading the whole archive), and a well-understood container format. Practical implications: you can inspect or edit EPUB contents with any ZIP tool or text editor, extract images with unzip, debug layout issues by opening the XHTML files in a browser, bulk-edit metadata by modifying content.opf directly, and create EPUBs programmatically by assembling the files and zipping them (with mimetype stored first, uncompressed).

EPUB is literally a renamed ZIP arquivo — rename any .epub to .zip e unzip it to see all the XHTML, CSS, images, e metadata arquivos inside. This design was intentional: ZIP fornece compressão, random access (extract individual arquivos sem reading the whole archive), e a well-understood container formato. Practical implications: you can inspect ou editar EPUB contents com any ZIP tool ou text editor, extract images com unzip, debug layout issues by abrindo the XHTML arquivos in um navegador, bulk-edit metadata by modifying content.opf directly, e create EPUBs programmatically by assembling os arquivos e zipping them (with mimetype stored first, não comprimido).

EPUB is literally a renamed ZIP Datei — rename any .epub to .zip und unzip it to see all the XHTML, CSS, images, und metadata Dateien inside. This design was intentional: ZIP bietet Komprimierung, random access (extract individual Dateien ohne reading the whole archive), und a well-understood Container Format. Practical implications: you can inspect oder bearbeiten EPUB contents mit any ZIP tool oder text editor, extract images mit unzip, debug layout issues by opening the XHTML Dateien in ein Browser, bulk-edit metadata by modifying content.opf directly, und create EPUBs programmatically by assembling die Dateis und zipping them (with mimetype stored first, uncompressed).

EPUB is literally a renamed ZIP archivo — rename any .epub to .zip y unzip it to see all the XHTML, CSS, images, y metadata archivos inside. This design was intentional: ZIP proporciona compresión, random access (extract individual archivos sin reading the whole archive), y a well-understood contenedor formato. Practical implications: you can inspect o editar EPUB contents con any ZIP tool o text editor, extract images con unzip, debug layout issues by opening the XHTML archivos in un navegador, bulk-edit metadata by modifying content.opf directly, y create EPUBs programmatically by assembling el archivos y zipping them (with mimetype stored first, uncompressed).

Send <strong>PDF</strong> when the document is final and the layout must be preserved exactly (contracts, invoices, certificates). Send <strong>DOCX</strong> when reviewers need to edit, comment, or track changes. Many teams send both: PDF as the canonical version + DOCX for editable feedback. PDF/A is the right pick for legal archival (ISO 19005).

Reflowable EPUB (the default) adapts to the reader's screen size, font size, and orientation — text wraps, images resize, and the reader can change the font. This is ideal for novels, essays, and textbooks. Fixed-layout EPUB defines a precise pixel canvas for each page — the reader renders it exactly as designed, similar to a PDF. Fixed layout is used for children's picture books, comic books, manga, cookbooks with complex layouts, and design-heavy content. Fixed layout trades readability on small screens for precise visual control. Most e-readers support both, but fixed layout requires more careful testing across device sizes.

Reflowable EPUB (the default) adapts para o reader's screen size, font size, e orientation — text wraps, images resize, e the reader can change the font. This is ideal para novels, essays, e textbooks. Fixed-layout EPUB defines a precise pixel canvas para each page — the reader renders it exactly as designed, similar a a PDF. Fixed layout is used para children's picture books, comic books, manga, cookbooks com complexo layouts, e design-heavy content. Fixed layout trades readability on small screens para precise visual control. Most e-readers support both, mas fixed layout requires more careful testing across device sizes.

Reflowable EPUB (the default) adapts zum reader's screen size, font size, und orientation — text wraps, images resize, und the reader can change the font. This is ideal für novels, essays, und textbooks. Fixed-layout EPUB defines a precise pixel canvas für each page — the reader renders it exactly as designed, ähnlich wie a PDF. Fixed layout is used für children's picture books, comic books, manga, cookbooks mit complex layouts, und design-heavy content. Fixed layout trades readability on small screens für precise visual control. Most e-readers support both, aber fixed layout requires more careful testing across device sizes.

Reflowable EPUB (the default) adapts al reader's screen size, font size, y orientation — text wraps, images resize, y the reader can change the font. This is ideal para novels, essays, y textbooks. Fixed-layout EPUB defines a precise pixel canvas para each page — the reader renders it exactly as designed, similar a a PDF. Fixed layout is used para children's picture books, comic books, manga, cookbooks con complex layouts, y design-heavy content. Fixed layout trades readability on small screens para precise visual control. Most e-readers support both, pero fixed layout requires more careful testing across device sizes.

Round-tripping between similar formats (DOCX ↔ ODT, DOCX → PDF) is generally safe. Round-tripping with format-specific features (Word macros, complex tables, footnotes) often loses fidelity. Embedded fonts survive only if both source and target support font embedding (PDF yes, DOCX yes, plain HTML no). Always preview the result before deleting the original.

Use EPUBCheck, the official W3C validator. Download epubcheck.jar from the W3C GitHub repository and run: java -jar epubcheck.jar your-book.epub. It checks the ZIP structure, OPF manifest and spine, navigation document, HTML/CSS validity, all referenced files, and accessibility metadata. A distributable EPUB should have 0 errors and ideally 0 warnings. Online alternatives include EPUB Validator at validator.w3.org/epub and the built-in validators in Sigil and Calibre. Major book distributors (Amazon, Apple, IngramSpark, Smashwords) also run EPUBCheck during submission — submitting an invalid EPUB will result in automatic rejection.

Use EPUBCheck, the official W3C validator. baixar epubcheck.jar de the W3C GitHub repository e run: java -jar epubcheck.jar your-book.epub. It checks the ZIP structure, OPF manifest e spine, navigation document, HTML/CSS validity, all referenced files, e accessibility metadata. A distributable EPUB should have 0 errors e ideally 0 warnings. Online alternatives include EPUB Validator at validator.w3.org/epub e the built-in validators in Sigil e Calibre. Major book distributors (Amazon, Apple, IngramSpark, Smashwords) also run EPUBCheck durante submission — submitting an invalid EPUB will result in automatic rejection.

Use EPUBCheck, the official W3C validator. herunterladen epubcheck.jar von the W3C GitHub repository und run: java -jar epubcheck.jar your-book.epub. It checks the ZIP structure, OPF manifest und spine, navigation document, HTML/CSS validity, all referenced files, und accessibility metadata. A distributable EPUB should have 0 errors und ideally 0 warnings. Online alternatives include EPUB Validator at validator.w3.org/epub und the built-in validators in Sigil und Calibre. Major book distributors (Amazon, Apple, IngramSpark, Smashwords) also run EPUBCheck während submission — submitting an invalid EPUB will result in automatic rejection.

Use EPUBCheck, the official W3C validator. descargar epubcheck.jar de the W3C GitHub repository y run: java -jar epubcheck.jar your-book.epub. It checks the ZIP structure, OPF manifest y spine, navigation document, HTML/CSS validity, all referenced files, y accessibility metadata. A distributable EPUB should have 0 errors y ideally 0 warnings. Online alternatives include EPUB Validator at validator.w3.org/epub y the built-in validators in Sigil y Calibre. Major book distributors (Amazon, Apple, IngramSpark, Smashwords) also run EPUBCheck durante submission — submitting an invalid EPUB will result in automatic rejection.

If the PDF contains real text (not scanned images), <code>pdftotext</code> from poppler-utils or <a href="/convert/pdf-to-txt">PDF to TXT</a> works in seconds. If the PDF is a scanned image, you need OCR — Tesseract is the open-source standard. KaijuConverter's PDF tools auto-detect text-vs-image PDFs and route accordingly.

For DOCX to EPUB: Pandoc is the best free tool — run "pandoc input.docx -o output.epub --epub-cover-image=cover.jpg" for a clean conversion that preserves headings, lists, and basic formatting. For more control, use Word to export as filtered HTML, then import into Sigil for cleanup. For PDF to EPUB: this is fundamentally lossy because PDF stores page layout, not document structure. Calibre can convert PDF to EPUB but the result often requires manual cleanup — especially for multi-column PDFs, tables, and PDFs without embedded text (scanned). The professional approach is OCR (Tesseract, Adobe Acrobat) to extract text, then reformat as EPUB from the extracted text.

For DOCX to EPUB: Pandoc is the best grátis tool — run "pandoc input.docx -o output.epub --epub-cover-image=cover.jpg" para a clean conversion that preserves headings, lists, e basic formatoting. para more control, usar Word to export as filtered HTML, then import em Sigil para cleanup. para PDF to EPUB: this is fundamentally com perdas because PDF stores page layout, not document structure. Calibre can converter PDF to EPUB mas the result often requires manual cleanup — especially para multi-column PDFs, tables, e PDFs sem embedded text (scanned). The profissional approach is OCR (Tesseract, Adobe Acrobat) to extract text, then reformat as EPUB de the extracted text.

For DOCX to EPUB: Pandoc is the best kostenlos tool — run "pandoc input.docx -o output.epub --epub-cover-image=cover.jpg" für a clean conversion that preserves headings, lists, und basic Formatting. für more control, verwenden Word to export as filtered HTML, then import in Sigil für cleanup. für PDF to EPUB: this is fundamentally verlustbehaftet because PDF stores page layout, not document structure. Calibre can umwandeln PDF to EPUB aber the result often requires manual cleanup — especially für multi-column PDFs, tables, und PDFs ohne embedded text (scanned). The professionell approach is OCR (Tesseract, Adobe Acrobat) to extract text, then reformat as EPUB von the extracted text.

For DOCX to EPUB: Pandoc is the best gratis tool — run "pandoc input.docx -o output.epub --epub-cover-image=cover.jpg" para a clean conversion that preserves headings, lists, y basic formatoting. para more control, usar Word to export as filtered HTML, then import en Sigil para cleanup. para PDF to EPUB: this is fundamentally con pérdidas because PDF stores page layout, not document structure. Calibre can convertir PDF to EPUB pero the result often requires manual cleanup — especially para multi-column PDFs, tables, y PDFs sin embedded text (scanned). The profesional approach is OCR (Tesseract, Adobe Acrobat) to extract text, then reformat as EPUB de the extracted text.

Light edits (annotations, signatures, form fields) are fine in any PDF reader. Structural edits (changing paragraphs, replacing images) are awkward — PDF is a presentation format, not an editing format. The robust workflow is: keep the source DOCX/MD/HTML as the master, regenerate the PDF when changes are needed. Tools that "edit PDFs" reverse-engineer the layout and frequently break it.