Guide · Updated 2026-11-12

Ebook formats explained: EPUB, PDF, MOBI, DRM and accessibility

What the common ebook file types are, why MOBI has faded, how DRM affects where you can read a book, what EPUB Accessibility 1.1 means, and how to choose the right file when an author or store offers several.

Ebooks come in a handful of file types. Which one you have decides where you can read it, how the text behaves on a small screen, and whether a screen reader can use it. This guide covers the formats you are most likely to see when an author or store offers you a choice.

EPUB: the open standard

EPUB (.epub) is the open, general-purpose ebook format. It is published by the W3C, the body that maintains web standards, and the current version is EPUB 3.3, the third major revision. The W3C copy opened for this guide is dated 13 January 2026. An EPUB is a ZIP-based package of web-style content (HTML or XHTML, CSS, images and fonts) plus a file that lists the contents and sets the reading order.

What matters for readers:

• Reflowable by default. The specification says EPUB content is meant to reflow to fit the screen, so you can change font size and the text rewraps. EPUB also allows fixed-layout books for picture books, comics and design-heavy titles, where every page keeps its position.
• Widely supported. NOOK, Kobo, Apple Books and Google Play Books all accept EPUB files you add yourself. Amazon’s Send to Kindle service now also accepts EPUB (see below).

For fiction and most nonfiction, EPUB is the best default.

PDF: fixed pages

A PDF keeps the exact page layout it was made with. That suits print replicas, workbooks, maps and anything where layout carries meaning. On a 6-inch e-reader, though, you usually have to zoom and pan, because the text doesn’t reflow the way it does in an EPUB. Every major reading platform covered on this site accepts PDFs. If an author offers both, choose the EPUB for reading and keep the PDF for reference.

MOBI and KF8: Amazon’s formats

Kindle has always used its own formats:

• MOBI (.mobi) was the older Kindle format, and many authors used to offer it as “the Kindle file.” Amazon has been retiring it. Kindle Direct Publishing stopped accepting MOBI for reflowable books on 1 August 2021 and for fixed-layout books on 18 March 2025. MOBI is also absent from Amazon’s current list of file types for Send to Kindle.
• KF8 (Kindle Format 8) was announced by Amazon in October 2011 alongside the Kindle Fire. It incorporates HTML5 and CSS3, which allowed richer formatting in Kindle books.
• General guidance: books bought in the Kindle Store are delivered by Amazon in its own formats, so readers don’t need to create or convert Kindle files themselves.

The practical upshot: to read a DRM-free book on a Kindle today, use the EPUB and Send to Kindle, which adds it to your Kindle library.

DRM: why some files won’t open

DRM (digital rights management) encrypts a book so it opens only in authorized software or devices. The main systems you will meet:

• Adobe DRM. Many non-Amazon stores and library platforms use Adobe’s ebook DRM. You often download a small .acsm file, which is a licence token rather than the book. Opening it in Adobe Digital Editions (ADE) downloads the actual EPUB or PDF, which you can then transfer to a supported e-reader such as a NOOK or Kobo. Adobe’s ebook platform is now maintained by Wipro, and ADE signs in with a ByteBooks ID instead of an Adobe ID (the switch was scheduled for 23 June 2026). If you already used an Adobe ID, using the same email address for your ByteBooks ID keeps your existing books available.
• Kindle DRM. Books bought in the Kindle Store are tied to your Amazon account and read in Kindle devices and apps. Apple notes that .azw files open only in their associated app.
• Readium LCP. LCP is a newer DRM for EPUB, PDF and audiobooks. It is run by EDRLab, a non-profit, and standardised as ISO/IEC 23078-2:2024. EDRLab describes it as needing no third-party account and as suited to library lending.

DRM limits where a file works. The NOOK app does not open third-party DRM EPUBs, and Send to Kindle rejects encrypted files. Files that authors give away directly are often DRM-free, which lets them go almost anywhere. If in doubt, ask the author. This guide doesn’t cover removing DRM.

Accessibility: EPUB Accessibility 1.1

The W3C’s EPUB Accessibility 1.1 is a W3C Recommendation. It was first published as one on 25 May 2023, and the current version is dated 17 October 2024. In short:

• Every conforming EPUB must include accessibility metadata (for example, whether it has text alternatives for images and any hazards such as flashing content). This lets stores and libraries tell readers what to expect before they download.
• An “accessible” EPUB is measured against the Web Content Accessibility Guidelines (WCAG 2). Level A is the minimum, and the spec strongly recommends Level AA.
• Publications with a print equivalent should include page navigation, in the form of a page list, so screen-reader users can follow page-numbered references such as citations or class discussions.
• Publishers can declare conformance with a statement such as “EPUB Accessibility 1.1 - WCAG 2.2 Level AA,” together with the name of whoever evaluated the book.

General guidance: reflowable EPUB is usually easier for assistive technology than PDF. If you rely on a screen reader, braille display or large text, ask for the EPUB.

How to pick a format

If a file ends in .acsm, you need Adobe Digital Editions first. If you only have a .mobi, ask the author or store for the EPUB.

For step-by-step device instructions, see our guide to loading an EPUB onto a NOOK and other readers.