Guides › The EPUB format

The EPUB format

What is inside an EPUB, how to edit, fix, validate and shrink one.

• EPUB vs PDF: Which Is Better for Reading?

  EPUB reflows to any screen size; PDF preserves exact layout. Here's when to use each format — and how to convert a PDF to EPUB in 30 seconds.

• EPUB 3 vs EPUB 2: What's the Difference and Why It Matters

  EPUB 3 adds HTML5, MathML, multimedia, and WCAG accessibility. EPUB 2 is obsolete but still common. Here's what changed, what broke, and which format you actually need.

• How to Change Font, Size, and Spacing in Your EPUB Reader (All Apps)

  Change font size, typeface, line spacing, and margins in Apple Books, Kindle, Kobo, Thorium, and Calibre. Plus why PDFs don't let you do this — and how to fix it.

• How to Edit an EPUB File — Sigil, Calibre, and VS Code

  Edit EPUB content, fix headings, correct metadata, and clean up styling using Sigil, Calibre's editor, or a plain text editor. When to edit manually vs use Premium Verification.

• EPUB vs MOBI: Which Format Should You Use? (MOBI Deprecated in 2022)

  Amazon deprecated MOBI in 2022 — Kindle now accepts EPUB natively. Here's what changed, why EPUB is the right format for everything, and what to do with old MOBI files.

• Best Ebook Format in 2026: EPUB, PDF, MOBI, or AZW3?

  Comparing every major ebook format: EPUB, PDF, MOBI, AZW3, FB2, and CBZ. Which is best for Kindle, Kobo, and reading apps — and when to convert between them.

• What Is an EPUB File? The Complete Guide to the EPUB Format

  EPUB is the universal ebook format used by Kobo, Apple Books, and now Kindle. Here's what EPUB files are, how they work, why they're better than PDF for reading, and how to open them.

• EPUB Not Opening? How to Fix Common EPUB Problems (2026)

  EPUB file won't open, shows blank pages, or has garbled text? Here are fixes for the most common EPUB problems on Kindle, Kobo, Apple Books, and reading apps.

• How to Edit EPUB Metadata — Title, Author, Cover, and ISBN (Calibre Guide)

  Edit EPUB metadata — title, author, series, cover image, ISBN, and language — using Calibre. Step-by-step for self-publishers preparing files for KDP, Kobo, and Apple Books.

• What Is EPUB DRM? Adobe DRM, Library Books, and How It Works (2026)

  EPUB DRM explained: what it is, how Adobe Digital Editions DRM works for library loans, why commercial Kindle books use a different system, and what DRM-free means.

• How to Convert EPUB to Audiobook — Text to Speech Guide (2026)

  Convert any EPUB to an audiobook using free and paid TTS tools: Balabolka, Speechify, ElevenLabs, and Apple Books' built-in reader. Step-by-step for each method.

• How to Add or Fix a Table of Contents in an EPUB (Calibre and Sigil)

  Add, rebuild, or fix the table of contents in any EPUB using Calibre's TOC editor or Sigil. Step-by-step for self-publishers and anyone fixing a broken EPUB navigation.

• Pandoc EPUB — Convert Markdown, HTML, and DOCX to EPUB (Complete Guide 2026)

  Use Pandoc to convert Markdown, HTML, or DOCX to EPUB from the command line. Covers metadata YAML, cover images, custom CSS, multi-chapter books, TOC generation, and EPUBCheck validation.

• How to Split an EPUB into Chapters or Smaller Files (Sigil, epubsplit, Calibre)

  Split a large EPUB into chapters or multiple volumes using Sigil, the epubsplit Python tool, or Calibre's editor. Covers file size limits, TOC rebuilding, and EPUBCheck validation.

• How to Reduce EPUB File Size — Image Compression, Font Subsetting, and More

  Reduce EPUB file size using Calibre lossless image compression, PNG-to-JPEG conversion, font subsetting with pyftsubset, and image resizing. Practical steps with size reduction estimates.

• How to Validate an EPUB — EPUBCheck, Ace by DAISY, and Online Validators

  Validate your EPUB before publishing to KDP, Apple Books, or Kobo. EPUBCheck catches spec errors, Ace checks accessibility. Includes common error codes and CI pipeline example.

• How to Repair a Broken or Corrupted EPUB File

  Fix broken EPUB files that won't open, show blank pages, or fail EPUBCheck. Step-by-step repair for missing files, broken TOC, malformed OPF, and corrupted ZIP structure.

• How to Add or Change the Cover Image in an EPUB (Calibre and Sigil)

  Add or replace an EPUB cover image using Calibre's metadata editor or Sigil. Covers OPF manifest properties, cover HTML page, platform size requirements, and why covers show as blank.

• EPUB Fonts — Embedding, Subsetting, and Licensing for E-Reader Distribution

  Embed custom fonts in EPUBs using @font-face, subset with pyftsubset to reduce file size, check font licensing with fsType, and apply IDPF obfuscation. Complete guide for publishers.

• Sigil EPUB Editor — Complete Guide for Editing and Publishing EPUBs

  Learn Sigil, the free open-source EPUB editor. Edit chapters in code or book view, find-replace across all files, generate TOC, edit OPF metadata, add fonts and images, split chapters.

• EPUB Annotations and Highlights — How They Work Across E-Readers

  How highlights, notes, and bookmarks work in Apple Books, Kindle, Kobo, and Thorium Reader. Where annotations are stored, how to export them, and cross-platform sync with Readwise.

• Best EPUB Editors — Free and Paid Tools Compared (2026)

  Compare Sigil, Calibre Edit Book, VS Code, Oxygen XML, Adobe InDesign, and Jutoh for EPUB editing. Honest strengths, weaknesses, and recommendations by use case.

• EPUB CSS — A Complete Styling Guide for E-Reader Publishing

  Style EPUBs with CSS that works across Kindle, Kobo, and Apple Books. Safe properties, what to avoid, drop caps, tables, and page breaks. Reader-specific notes for each platform.

• EPUB Images: Formats, Compression, Alt Text, and Cover Best Practices

  How to handle images in EPUB: supported formats, recommended dimensions, compression before embedding, alt text for accessibility, and cover image setup.

• Inside an EPUB File: Structure, Files, and How It All Works

  How an EPUB is structured: the ZIP archive layout, mimetype file, container.xml, OPF package document, manifest, spine, and XHTML content files explained.

• EPUB Footnotes and Endnotes: Popup Notes and Best Practices

  How to implement footnotes in EPUB 3: popup notes with epub:type noteref and footnote, separate notes files, endnotes vs footnotes, and accessibility with DPUB-ARIA roles.

• Fixed-Layout EPUB (FXL): When to Use It and How It Works

  Guide to fixed-layout EPUB: rendition:layout pre-paginated, viewport dimensions, two-page spreads, accessibility challenges, and reader support comparison.

• Multilingual EPUB: How to Handle Multiple Languages in One Ebook

  Bilingual books, glossaries, and mixed-language EPUBs require correct language tagging at the document and element level. Here's how to structure multilingual EPUB content correctly.

• How to Export EPUB from Adobe InDesign — Settings and Fixes (2026)

  Export EPUB 2 and EPUB 3 from Adobe InDesign. Which settings matter, common export problems and how to fix them, and when to post-process in Sigil or Calibre.

• EPUB Dark Mode — How to Enable and Customise Dark Reading on Kindle, Kobo, and Reading Apps (2026)

  EPUB dark mode inverts the page to white-on-black or uses a dark grey theme to reduce eye strain in low light. Every major e-reader and reading app supports it — here's how to enable it and why EPUB works better than PDF in dark mode.

Other sections: PDF to EPUB · Kindle, Kobo and e-readers · Reading apps · Research and study · Accessibility and the EAA · Other conversions