Ebook formats are a more confusing topic than they should be. There are five formats that account for almost all ebooks in circulation (EPUB, EPUB3, AZW3, KFX, PDF), one that is functionally dead but still encountered (MOBI), and a handful of legacy or niche formats (PRC, FB2, LIT, LRF) that come up only with old files. Add DRM (digital rights management) on top of every commercial format and the question “can I read this on my device” stops having an obvious answer. This guide walks through what each format actually is, what it does and does not support, which devices read which, and how to convert between formats when you need to.
EPUB: the open standard
EPUB (Electronic Publication) is an open standard from the International Digital Publishing Forum, now maintained by the W3C. It is the default format for almost every ebook sold outside the Amazon ecosystem.
Technical basics. An EPUB file is a renamed ZIP archive containing HTML, CSS, images, fonts, and a metadata manifest. The text reflows to fit any screen size and supports user-controlled fonts, sizes, margins, and spacing.
Current version. EPUB3, with EPUB 3.3 the latest major revision (2023). Adds HTML5 support, fixed-layout pages for comics and children’s books, embedded audio and video, MathML, accessibility features (read-aloud, navigation), and improved internationalization (right-to-left languages, vertical text).
Devices that read EPUB natively. Kobo, PocketBook, Boox, Tolino, Onyx, every iOS app (Apple Books, Calibre Companion, KyBook), every Android app (Moon+ Reader, Lithium, Calibre app). Notably absent: Kindle, which requires conversion through Send to Kindle.
DRM on EPUB. Adobe ADEPT DRM is the most common commercial DRM on EPUBs, used by Kobo, library lending through OverDrive (Libby), and many smaller stores. Some publishers also use LCP (Readium Licensed Content Protection). DRM-free EPUBs from Smashwords, Standard Ebooks, and many indie publishers play in any reader without restriction.
AZW3 and KFX: Amazon’s stack
Amazon’s ebook formats are descended from Mobipocket, an early ebook standard Amazon acquired in 2005.
AZW. The original Kindle format, essentially Mobipocket with Amazon-specific DRM. Used on Kindle from 2007 to about 2011.
AZW3 (KF8). Introduced 2011 with the Kindle Fire. Adds HTML5 and CSS3 support, fixed-layout pages, embedded fonts, and better typography. Often called KF8 (Kindle Format 8).
KFX. Introduced 2015. Amazon’s current format, used on all modern Kindles. Adds advanced typography (enhanced word and character spacing, kerning, hyphenation), the Bookerly font, X-Ray (popups linking characters and concepts), and Page Flip. KFX files are downloaded fresh from Amazon servers and contain device-specific encryption tied to your account.
Compatibility. KFX plays only on Kindle devices and the Kindle app. AZW3 plays on Kindle and, with Calibre and the right plugins, on third-party software. Neither plays on Kobo, PocketBook, or Boox.
DRM. All commercial AZW3 and KFX files have Amazon DRM, tied to your account. Amazon-published DRM-free content does exist for some titles (mostly indie KDP books that opt out of DRM at publishing) but is the exception, not the rule.
MOBI: the deprecated grandparent
MOBI (Mobipocket) was the foundation of every Kindle format up to AZW3. Amazon retired Mobipocket-style files in 2022.
Current status. Send to Kindle stopped accepting MOBI uploads in late 2022. Kindle Direct Publishing stopped accepting MOBI uploads in August 2022. Old MOBI files still work on older Kindles but are not actively supported.
What to do with a MOBI library. Convert to EPUB or AZW3 with Calibre. Conversion is fast (seconds per book) and the result is forward-compatible. Keep the original MOBI as a backup only.
PDF: documents, not books
PDF (Portable Document Format) from Adobe is the default for documents, papers, manuals, and scanned books. It is not designed as an ebook format.
What PDF does well.
- Preserves exact layout, fonts, and formatting.
- Excellent for academic papers, manuals, and reference works that need a fixed layout.
- Universal compatibility; every device reads PDF.
What PDF does badly.
- Does not reflow text to fit screen size. A PDF made for A4 paper looks tiny on a 6-inch e-reader.
- No user control of fonts, sizes, or margins.
- Larger file size than EPUB for the same content.
- Slow rendering on e-ink devices.
For book-format PDFs: convert to EPUB with Calibre. The conversion works well for text-source PDFs (manuscripts, articles, books generated from Word or LaTeX) and poorly for scanned-image PDFs without OCR.
Less common formats you may encounter
FB2 (FictionBook). A Russian-developed XML-based format. Common in Eastern European book pirate libraries and some indie sites. Calibre and many readers support it.
LIT (Microsoft Reader). Discontinued in 2012. Convert to EPUB and forget.
LRF (BBeBook). Sony’s old format. Discontinued. Convert to EPUB.
PRC. A variant of MOBI / Palm Doc. Convert to EPUB.
TXT. Plain text. Useful for very simple ebooks (Project Gutenberg often offers it as a fallback). Almost all readers handle it; reformatting on import is sometimes ugly.
HTML and HTMLZ. Calibre can import and convert.
Conversion: Calibre is the answer
Calibre (free, open-source) is the de facto tool for ebook conversion and library management. It handles every format mentioned above, runs on Windows, Mac, and Linux, and is actively maintained.
Common conversions.
- MOBI to EPUB: lossless and fast.
- EPUB to AZW3: necessary for old Kindles that do not support EPUB through Send to Kindle.
- PDF to EPUB: works well for text-source PDFs, poorly for scanned PDFs without OCR.
- Anything to AZW3 or EPUB for forward compatibility.
Tips.
- Always keep the original file as a backup before converting.
- The “Heuristic processing” option in Calibre fixes common formatting issues during conversion.
- For PDFs, the “PDF Output” tab in Calibre’s conversion settings includes important options around page detection and table handling.
- Embed fonts in EPUB only if the source has unusual typography that matters; otherwise default fonts work better across devices.
Format compatibility quick reference
| Device or app | EPUB | EPUB3 | AZW3 | KFX | MOBI | |
|---|---|---|---|---|---|---|
| Kindle (any modern model) | Via Send to Kindle (converted) | Same | Yes | Yes | Legacy only | Yes (poor) |
| Kobo | Native | Native | No | No | No | Yes |
| Boox / Onyx | Native | Native | No | No | Some | Yes |
| Apple Books (iOS) | Native | Native | No | No | No | Yes |
| Google Play Books | Native | Native | No | No | No | Yes |
| Calibre | Yes | Yes | Yes | Yes (with plugin) | Yes | Yes |
Which format to buy and store
For DRM-free purchases and personal library backups, EPUB is the format to buy and keep. It is the most widely compatible, the most actively maintained, and the easiest to convert to anything else. Buy from retailers that sell DRM-free EPUBs whenever possible (Standard Ebooks, Smashwords, Itch.io for indie, Tor.com on DRM-free sales, many indie publisher direct sales).
For Kindle library, you are locked into AZW3 or KFX whether you like it or not. Accept the lock-in or move to Kobo for format freedom. See the Kindle vs Kobo vs Paperwhite breakdown for the ecosystem decision and the e-ink vs tablet reading guide for whether a dedicated reader is the right call at all.
Frequently asked questions
What is the difference between EPUB and EPUB3?+
EPUB2 (released 2007) supports reflowable text, basic styling, and embedded fonts. EPUB3 (2011 onward) adds HTML5, CSS3, embedded audio and video, fixed-layout pages for comics and children's books, and accessibility features like read-aloud. Most ebooks sold in 2026 are EPUB3, but many older library and Project Gutenberg titles are still EPUB2. Modern e-readers and apps handle both transparently.
Is MOBI still used anywhere in 2026?+
Effectively no. Amazon stopped accepting MOBI uploads for Kindle Direct Publishing in August 2022, and the Send to Kindle service stopped accepting MOBI files in late 2022. Old MOBI files still work on older Kindles but are not the standard format anywhere. If you have a MOBI library from before 2022, convert to EPUB for forward compatibility. Calibre handles this in seconds.
Why does Amazon use proprietary formats instead of EPUB?+
Lock-in. Amazon's AZW3 (a refined Mobipocket descendant) and KFX (introduced in 2015) are technically capable formats with strong typography, embedded fonts, and X-Ray annotations, but their proprietary DRM keeps Kindle books from playing on Kobo, Pocketbook, or any other ecosystem. Amazon added EPUB support to Send to Kindle in 2022, but the files are converted to KFX on Amazon's servers before download. You cannot read a native EPUB on a Kindle.
Can I convert a PDF to EPUB and have it work well?+
It depends on the PDF. A reflowable PDF made from a text source (Word, LaTeX, plain manuscript) converts to EPUB cleanly with Calibre. A scanned PDF made from images of book pages converts poorly because each page becomes an image with no real text underneath. OCR (optical character recognition) can extract text from scanned PDFs, with accuracy of 90 to 99 percent depending on the source quality, then convert to EPUB. ABBYY FineReader, Adobe Acrobat Pro, and Tesseract are common OCR tools.
What is the safest way to back up ebooks I have purchased?+
Buy from DRM-free retailers when possible (Smashwords, Standard Ebooks, Project Gutenberg, many indie publishers, Tor.com when on sale). Download the EPUB or PDF directly and back it up to your own storage. For DRM-protected books from Amazon, Apple, or Kobo, stripping DRM violates the terms of service in most jurisdictions, though local law on personal-use backups varies. The legally safe approach is to maintain a separate library of DRM-free purchases and rely on the storefront to re-download DRM-locked titles.
