Ebook Formats Explained — EPUB, MOBI, PDF, AZW3, KFX & More
The ebook world has a format problem. Kindle uses its own formats. Kobo and Apple Books use EPUB. Some books are only available as PDF. You buy a book on one platform and cannot read it on another. This guide explains every major ebook format, which devices support which formats, and how to convert between them so your books work everywhere.
EPUB — The Open Standard
EPUB (Electronic Publication) is the industry-standard ebook format, maintained by the W3C. It is an open format based on HTML and CSS — essentially a packaged website. EPUB supports reflowable text (content adjusts to screen size), embedded fonts, images, table of contents, metadata, and accessibility features. Text reflows beautifully on any screen, from a 6-inch e-reader to a 13-inch tablet.
EPUB is supported by Kobo, Apple Books, Google Play Books, Nook, and virtually every e-reader and reading app except Amazon Kindle. If you are publishing an ebook and want it to work on the widest range of devices (excluding Kindle), EPUB is the format to produce. It is also the primary format used by libraries (OverDrive, Libby) for lending digital books.
MOBI and AZW3 — Amazon Kindle Formats
MOBI is Amazon's original Kindle format, based on the old Mobipocket standard. It works on all Kindle devices but is technically outdated — it has limited formatting support, no fixed-layout capability, and is being phased out. Amazon stopped accepting MOBI for new Kindle Direct Publishing (KDP) uploads in 2023.
AZW3 (also called KF8 — Kindle Format 8) is the modern Kindle format. It supports advanced formatting, embedded fonts, HTML5 and CSS3 features, and fixed-layout content. AZW3 is what Kindle devices actually use to render books today. When you buy a book from the Kindle Store, it downloads in KFX or AZW3 format with DRM applied.
In 2024, Amazon began accepting EPUB uploads directly through KDP, converting them to Kindle's internal format. This was a major shift — previously, you had to upload MOBI or DOC/DOCX and let Amazon convert. You can now upload EPUB to KDP and it works correctly in most cases.
PDF — Fixed Layout, Not Ideal for E-Readers
PDF preserves exact layout and formatting, which makes it perfect for print but problematic for e-readers. A PDF designed for an A4 page looks tiny on a 6-inch Kindle screen because the text does not reflow — it displays the entire page scaled down. You end up zooming and panning constantly. PDFs work well on tablets and computers where the screen is large enough to display the page at a readable size.
Use PDF for ebooks that depend on exact layout: textbooks with complex diagrams, cookbooks with formatted recipes and photos, technical manuals, and art books. For text-heavy books (novels, non-fiction, biographies), EPUB is always the better choice because it reflows to fit any screen.
FB2 — Popular in Russia and Eastern Europe
FB2 (FictionBook) is an XML-based ebook format widely used in Russia and Eastern European countries. It supports structured metadata, embedded images, and reflowable text. Major Russian ebook stores (LitRes, Bookmate) distribute in FB2. If you encounter FB2 files, most reading apps can open them, or you can convert to EPUB for broader compatibility.
DRM — Why You Cannot Always Convert
DRM (Digital Rights Management) is copy protection applied by publishers and retailers. Books purchased from the Kindle Store, Apple Books, Kobo, and Nook are typically DRM-protected. DRM-protected ebooks cannot be freely converted between formats — you can only read them in the retailer's own app or on their approved devices. DRM-free ebooks (common from independent authors, Smashwords, and some publishers) can be converted freely.
When purchasing ebooks, look for DRM-free options when available. Many indie publishers and authors sell DRM-free EPUB files directly from their websites. Tor Books, one of the largest science fiction publishers, sells all ebooks DRM-free.
Converting Between Formats
EPUB to MOBI/AZW3: Use an ebook converter or Calibre (desktop software). This is the most common conversion — people who buy EPUB books and want to read them on Kindle. PDF to EPUB: Possible but imperfect. Text-heavy PDFs convert reasonably well. Layout-heavy PDFs (columns, images, tables) produce poor results because PDF layout cannot be reliably converted to reflowable HTML. MOBI to EPUB: Clean conversion if the MOBI file is DRM-free.
Which Format to Use
Reading on Kindle: AZW3 or EPUB (via Send to Kindle). Reading on Kobo, Apple, Nook: EPUB. Publishing on KDP: EPUB or DOCX upload. Self-hosting and selling direct: EPUB (DRM-free). Print-layout books (textbooks, art books): PDF. Maximum compatibility across all readers: Produce both EPUB and PDF.
Convert Ebook Files
Convert between EPUB, MOBI, AZW3, PDF, and more — free, in your browser.