# What is EPUB 3 and why does it matter?

Source: https://epubcc.com/answers/what-is-epub-3/ (updated 2026-11-04)

EPUB 3 is the current version of the open EPUB e-book standard: a ZIP file holding XHTML chapters, CSS, images and a package document, which reading apps re-flow for any screen. It matters because it is the one format nearly every reader and store accepts, and its latest revision, EPUB 3.3, is a W3C Recommendation.

## What is inside an EPUB 3

- A mimetype file and a META-INF/container.xml that tell apps where the book starts.
- A package document (OPF) with the metadata, the list of every file (the manifest) and the reading order (the spine).
- XHTML content documents for the chapters, plus CSS, images and fonts.
- A navigation document: the table of contents, and optionally a page list and landmarks.

## What EPUB 3 adds over EPUB 2

- HTML5 markup instead of XHTML 1.1, with semantic tags for footnotes and other structures (epub:type).
- Right-to-left and vertical writing, and MathML for equations.
- Accessibility metadata, and page lists that map to the printed pages of the source.
- Media overlays (text synchronised with audio) and fixed-layout pages when needed.

## Why it matters for a converted PDF

Producing EPUB 3 means one file for Kobo, Apple Books, Google Play Books and most apps, and for Kindle through Send to Kindle. Validators such as EPUBCheck check files against the EPUB 3.3 rules, which gives a clear test of whether the file is well made. EpubCC writes EPUB 3, with footnotes as EPUB 3 notes linked from their calls and right-to-left books marked with the right page direction.

## EPUB 3 and the older formats

EPUB 2 files (the 2.0.1 standard dates from 2010) still open in most apps, and Amazon's Kindle formats (AZW3, KFX) are proprietary. There is no reason to produce either from a PDF today: EPUB 3 is the open standard, and Kindle accepts it through Send to Kindle.

**Can old e-readers open EPUB 3?** Most current readers do. An EPUB 3 can also include an EPUB 2 table of contents (NCX) so that very old reading systems can still navigate it.

**Is EPUB 3 the same as EPUB 3.3?** EPUB 3.3 is the latest revision of EPUB 3. A well-made EPUB 3 file validates against the 3.3 rules.