The ebook format that matters most in 2026 is EPUB. It's an open W3C standard, every major store takes it as an upload, and every reading app except Amazon's reads it directly. Kindle devices use Amazon's own formats, KFX and the older AZW3, but nobody uploads those: Amazon builds them from the EPUB or Word file you send. PDF is the fixed-page option. MOBI, the old Kindle format, is retired on both the publishing and the reading side.
Which file you need depends on whether you're reading a book or publishing one, so this page answers both. Every store rule below comes from that store's own help pages, linked beside it, as of October 2026.
The Formats at a Glance
| Format | Extension | What it is | Read on | Can an author upload it? |
|---|---|---|---|---|
| EPUB | .epub | Open standard, reflowable or fixed layout. Current version EPUB 3.3 | Apple Books, Google Play Books, Kobo, Nook, most apps; Kindle after Send to Kindle converts it | Yes, at every major store |
| Fixed pages, looks identical everywhere | Everything, though small screens make you zoom | Google Play; KDP in nine languages; not Apple, Kobo or Draft2Digital | ||
| Word | .docx, .doc | Manuscript file, not a reading format | Not read by e-readers | KDP, Kobo, Draft2Digital, B&N Press convert it for you |
| KPF | .kpf | Kindle Package Format, exported by Kindle Create | Not a reading format | KDP only |
| KFX | .kfx | Amazon's current Kindle format (Kindle Format 10, 2015) | Newer Kindles and Kindle apps | No, Amazon generates it |
| AZW3 (KF8) | .azw3 | Kindle Format 8, from 2011 | Older Kindles; sideloading | No |
| MOBI | .mobi, .prc | Legacy Mobipocket format the first Kindles used | Kindles by USB only | Not at KDP since 2021 (reflowable) and 2025 (fixed layout); Kobo still converts it |
If You're Reading: Which File Opens Where
For a reader, the format question is really a device question. Kindle is the exception; everything else reads EPUB.
Kindle: EPUB yes, MOBI no
Amazon's Send to Kindle help page lists the files it accepts today: EPUB, PDF, TXT, RTF, HTML (.html, .htm) and images (JPEG, GIF, PNG, BMP). Files sent through the web uploader can be up to 200 MB. Send to Kindle converts an EPUB into Kindle's own format on the way, which is why a Kindle can read a book bought from another store as long as it has no DRM.
MOBI is gone from that list. Amazon began winding down MOBI, AZW and PRC files on Send to Kindle on November 1, 2023 and ended support on December 20, 2023, according to its notice to users as reported by The eBook Reader. Word files are also missing from the current list, although older guides still say they work. So the short answer to "does Kindle prefer EPUB or MOBI" is EPUB.
PDFs on a Kindle
A Kindle opens PDFs, and Send to Kindle accepts them. The catch is that a PDF keeps its page size. A letter-size page on a 6-inch screen means small text or a lot of zooming. PDF suits documents with a fixed design. For a novel, an EPUB reflows to the screen and reads far better.
Everything else: EPUB
Apple Books, Google Play Books, Kobo and Nook all read EPUB natively. On a computer, most reading apps open EPUB, and calibre can open almost anything. A store copy with DRM opens only in that store's app, whatever the file format.
New in 2026: Kindle Books You Can Download as EPUB
Since January 20, 2026, a buyer of a DRM-free Kindle book can download it as EPUB and PDF, under a KDP policy that most format guides haven't caught up with yet. Kindle+ (formerly Kindle Unlimited) borrowers can't, and books that use DRM aren't affected.
For authors, the detail that matters is the date. Books published after December 9, 2025 without DRM get the download automatically. Books published before that date need the author to opt in, one book at a time: in KDP, edit the eBook content and, under DRM, choose the option that allows a PDF or EPUB download, then republish. It takes 24 to 72 hours. If an author later turns DRM back on, new downloads stop and copies already downloaded stay with the buyers. Royalties don't change either way.
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If You're Publishing: What Each Store Accepts
| Store | Ebook uploads accepted | Fixed layout | Notes |
|---|---|---|---|
| Amazon KDP | EPUB, Word (DOC/DOCX), KPF; also HTML, RTF, TXT and PDF | Yes, as EPUB or KPF | PDF ebooks only in English, French, German, Italian, Portuguese, Spanish, Catalan, Galician and Basque. MOBI not accepted |
| Apple Books | EPUB 3 only | Yes, whole-book setting | Every book must pass Apple's Transporter validation. Word users export EPUB from Pages |
| Google Play Books | EPUB and PDF | Yes, in EPUB 2 and 3 | No Word or HTML. Google recommends uploading both an EPUB and a PDF. EPUB 3.3 preferred |
| Kobo Writing Life | DOC/DOCX, ODT, EPUB, MOBI | Kobo sells fixed-layout titles | Everything is converted to Kobo's own EPUB; files up to 100 MB |
| Barnes & Noble Press | EPUB, Word, HTML, TXT | Not documented | Ebook files must be under 20 MB; B&N advises EPUB for heavily formatted books |
| Draft2Digital | DOC/DOCX, RTF, ODT, TXT, EPUB | Not documented; its FAQ treats ebooks as reflowable | EPUB accepted for ebooks only, not for print. Files under 100 MB. No PDF uploads |
Sources: KDP supported formats; Apple Books Asset Guide 5.3.1; Google Play Books file requirements; Kobo Writing Life file types; B&N Press file size; Draft2Digital FAQ.
A Word file is the easiest upload, and the store's converter decides how it looks. An EPUB you built and checked yourself looks the way you made it at every store. That's why most self-publishers who sell in more than one place keep one validated EPUB 3 file as their master.
Amazon's Formats, Untangled
Amazon's format names cause most of the confusion around ebooks, because readers see some of them and authors see others. Here's the whole family in order:
- AZW (2007). The format of the first Kindle, built on Mobipocket's MOBI.
- AZW3, also called KF8 (2011). Introduced with the Kindle Fire, with support for a subset of HTML5 and CSS3. Older Kindles still receive this version of a book.
- KFX, or Kindle Format 10 (2026). Carries Enhanced Typesetting: hyphenation, kerning, ligatures, better image handling. Amazon applies it automatically when a book qualifies, and KDP says "customers who use older devices receive a KF8 version of the title" (KDP, Enhanced Typesetting).
- KPF. The publishing file Kindle Create exports for upload to KDP. Kindle Create can also export EPUB, for reflowable books only.
- MOBI. Retired. KDP stopped accepting reflowable MOBI files on August 1, 2021 and fixed-layout MOBI on March 18, 2025 (KDP MOBI FAQ).
The author uploads EPUB, Word or KPF. Amazon turns it into KFX or AZW3 for each reader's device. Neither of those is a file an author ever makes.
Reflowable vs. Fixed Layout
A reflowable ebook has no fixed pages. The reader picks the font size, and the text rewraps to fit the screen. Novels, memoirs and most nonfiction should be reflowable.
A fixed-layout ebook keeps every page exactly as designed, like a PDF inside an EPUB. Picture books, comics, cookbooks and anything where text sits on top of images need it. Apple, Google and KDP all accept fixed-layout EPUB. For a children's book or comic on Kindle, Kindle Create can build one from a PDF or a set of images. Draft2Digital and B&N Press don't document fixed-layout uploads, so a picture book is safer going to those stores directly from Apple, Google or KDP.
A fixed-layout book reads badly on a phone, and some store features skip it: Kobo, for example, shows no Instant Preview for fixed-layout titles. Use it only when the design needs it.
EPUB Versions and Checking Your File
EPUB 3.3 is the current standard. The W3C first published it as a Recommendation on May 25, 2023, and the latest edition is dated January 13, 2026 (W3C, EPUB 3.3). EPUB 3.4 is still a draft. Apple's guide covers EPUB 3 only, and Google prefers 3.3 while still accepting EPUB 2. An EPUB 3 file works everywhere.
Check the file before you upload it:
- EPUBCheck, the official free validator maintained by the DAISY Consortium for the W3C, reports structural errors in any EPUB. Kobo's guidelines warn that files with errors won't be accepted into OverDrive's library catalogue, and Google recommends running it.
- Kindle Previewer, Amazon's free desktop app, shows the book as each Kindle device will display it and whether Enhanced Typesetting is on. It opens EPUB, Word (English content only), HTML and KPF.
- Apple Transporter runs Apple's own validation, and every book has to pass it.
Making and Converting the Files
Most authors write in Word or Google Docs and need an EPUB at the end. The common routes:
- Vellum (Mac) and Atticus format the book once and export both the ebook and the print PDF.
- Kindle Create is free and exports KPF for KDP, plus a reflowable EPUB.
- Reedsy Studio is a free browser editor with EPUB export.
- Draft2Digital's formatter turns a Word file into an EPUB you can download and use elsewhere.
- Sigil edits the EPUB itself, for people comfortable with its HTML.
- calibre converts between dozens of formats (EPUB, AZW3, MOBI, DOCX, PDF and more), but it can't open DRM-protected store files or output KFX or KPF. Use it for your own reading files.
For the print edition you'll need a PDF interior and a cover sized to your page count. Our words-to-pages calculator estimates the page count from your manuscript, and the KDP cover calculator gives the spine width and full cover size for it.
Other Extensions You Might See
- .kepub.epub: Kobo's delivery version of EPUB, which Kobo makes from your upload.
- .cbz and .cbr: comic book archives, a zipped folder of page images.
- .fb2: FictionBook, an XML format common in Russian-language ebook collections.
- .djvu: scanned documents, often old books in archives.
- .iba: iBooks Author projects. Apple says iBooks Author "is no longer updated or available," and its files open in Pages (Apple Support).
- .mp3, .m4b: audiobooks, which are a separate world; ACX, for example, wants 192 kbps MP3 files. See audiobook formats.
Questions People Ask
What is the best file format for ebooks?
EPUB 3. Every major store accepts it, it reflows on any screen, and it converts cleanly to Kindle. Keep PDF for books that must keep a fixed page design.
Does Kindle use EPUB or MOBI?
Neither, internally: Kindles read KFX and AZW3. But EPUB is what you send to one. Send to Kindle accepts EPUB and converts it, and it stopped accepting MOBI in December 2023. KDP takes EPUB uploads and no longer takes MOBI.
What's the difference between EPUB and an ebook?
An ebook is any digital book. EPUB is one file format for it, the open standard most stores and apps use. A Kindle ebook is the same book in Amazon's format.
What will open an EPUB file?
Apple Books on a Mac, iPhone or iPad; Google Play Books; Kobo and Nook readers; calibre and most reading apps on Windows and Android; and a Kindle, once the file has gone through Send to Kindle.
What format does Barnes & Noble use for ebooks?
Nook reads EPUB. Authors can upload EPUB, Word, HTML or TXT through B&N Press, with ebook files under 20 MB.
Can I upload a PDF to KDP as an ebook?
Yes, for books in nine languages, including English and Spanish, but it's the format least likely to convert well into a reflowable Kindle book. KDP lists EPUB, Word and KPF as its recommended formats. Kindle Create can turn a PDF into a "Print Replica" ebook that keeps the page design.
