eBook Formats Explained: EPUB, MOBI, PDF and More
What You'll Learn in This Guide
The eBook format landscape is more fragmented than any other file type category. Amazon uses one format, Apple uses another, and everyone else uses a third. This guide explains every major eBook format, why this fragmentation exists, and how to move your books between devices and platforms.
You will learn which formats work on which devices, how DRM restricts your purchased books, and the best format choices for both reading and self-publishing. We include links to free conversion tools and step-by-step guides throughout.
Understanding eBook Formats: Reflowable vs Fixed Layout
eBook formats split into two categories based on how they handle page layout. Reflowable formats (EPUB, MOBI, AZW3) adjust text to fit your screen size and chosen font size. If you increase the font on your e-reader, the text reflows — words per line decrease, and you get more pages. This is what most readers expect from an eBook.
Fixed-layout formats (PDF, some EPUB 3, comic book formats) maintain exact page positioning regardless of screen size. This works well for content where layout matters — textbooks with diagrams, cookbooks with photos beside recipes, and comics. But it creates a poor experience on small screens because you end up zooming and panning instead of just reading.
For text-heavy books like novels, biographies, and most non-fiction, reflowable is always better. For visually rich content, fixed layout preserves the intended design. This distinction is the single most important factor in choosing an eBook format.
EPUB — The Open Standard
EPUB (Electronic Publication) is the international open standard for eBooks, maintained by the W3C. It is used by Apple Books, Google Play Books, Kobo, Barnes and Noble Nook, and virtually every library lending system worldwide. If you buy an eBook from anywhere other than Amazon, it is almost certainly EPUB.
EPUB 3, the current version, is essentially a packaged website — it uses HTML, CSS, and JavaScript internally. This means EPUB 3 supports embedded audio, video, interactive elements, MathML for equations, and advanced typography. In practice, most eBooks use only basic text and image features, but the capability is there for enhanced publications.
The one major platform that historically did not support EPUB was Amazon Kindle. However, Amazon added EPUB support for Send to Kindle in 2022, meaning you can now send EPUB files to your Kindle for personal reading. For publishing on the Kindle store, Amazon still converts your files to their proprietary format internally.
MOBI — The Kindle Legacy Format
MOBI (Mobipocket) was Amazon's original Kindle format, acquired when they bought Mobipocket in 2005. MOBI supports basic formatting, images, and DRM but lacks many modern features — no embedded fonts, no audio/video, limited CSS support, and no fixed layout option.
Amazon has been phasing out MOBI in favor of KF8/AZW3 for years. In 2023, Amazon stopped accepting MOBI files for Send to Kindle. If you have MOBI files in your collection, they still work on Kindles, but for new books, EPUB or AZW3 is the better choice.
AZW3/KF8 — Amazon's Modern Format
AZW3 (also called KF8 or Kindle Format 8) is Amazon's current eBook format. Internally, it is very similar to EPUB 3 — based on HTML5 and CSS3 — but wrapped in Amazon's proprietary container with their DRM system. When you buy a book from the Kindle store, you receive an AZW3 file.
AZW3 supports embedded fonts, fixed layout, enhanced typesetting, and all the features readers expect from modern eBooks. The catch is that AZW3 files with DRM only work on Kindle devices and apps. Without DRM, you can convert AZW3 to EPUB or other formats using tools like Calibre.
PDF as an eBook
PDF is technically readable on every e-reader, tablet, and phone, but it makes a poor eBook format for most content. Because PDF uses fixed layout, text does not reflow when you change font size. On a 6-inch Kindle screen, a letter-size PDF requires constant zooming and panning to read — an exhausting experience for anything longer than a few pages.
PDF works well as an eBook format only for content that requires fixed layout: academic papers with complex equations, technical manuals with precise diagrams, and sheet music. For standard books, always prefer EPUB or AZW3. If you only have a PDF version of a book, converting to EPUB can improve the reading experience dramatically — though the conversion quality depends on the PDF structure.
FB2 — FictionBook
FB2 (FictionBook) is an XML-based eBook format that is extremely popular in Russia and Eastern Europe but rarely seen elsewhere. It has excellent metadata support, stores structural information about the book cleanly, and several popular reading apps support it natively.
If you encounter FB2 files and want to read them on mainstream devices, converting to EPUB is straightforward and preserves formatting well. For English-language readers and publishers, FB2 is not a format you need to worry about.
CBR/CBZ — Comic Book Formats
CBR and CBZ are specialized formats for digital comics and manga. CBR is a RAR archive of images, and CBZ is a ZIP archive of images — the only difference is the compression format used. Each page of the comic is stored as a separate image file (usually JPG or PNG) inside the archive.
Dedicated comic reader apps handle CBR and CBZ with features like page-turn animations, panel zoom, and reading direction settings for manga. For a detailed comparison of comic book formats including PDF alternatives, see our CBR vs CBZ vs PDF comic format guide.
Kindle vs Everything Else
Amazon's Kindle ecosystem is the elephant in the eBook room. When you buy a Kindle book, it is locked to Amazon's apps and devices with DRM. You cannot read it in Apple Books, Google Play Books, or Kobo. This lock-in is intentional — it keeps you buying Kindle devices and shopping at Amazon.
The rest of the eBook world largely uses EPUB with Adobe DRM or no DRM at all. Books from Apple Books, Google Play, Kobo, and most independent publishers use EPUB. Library lending systems like OverDrive/Libby also use EPUB. If you want maximum flexibility with your book collection, EPUB is the format that gives you the most options.
EPUB vs MOBI vs PDF: Which to Choose
For reading on Kindle devices, AZW3 is the native format. For reading on everything else (Apple, Kobo, Google, Nook), EPUB is the standard. For documents that must maintain exact layout, PDF is the right choice. For personal book collections that you want to read on any device, EPUB is the most versatile option.
For a detailed comparison with conversion tips, see our EPUB vs MOBI vs PDF vs AZW3 format guide.
Calibre vs Online Converters
Calibre is the gold standard desktop application for eBook management and conversion. It handles virtually every eBook format, offers extensive customization options, and manages your entire library with metadata editing, cover art, and device syncing. For power users with large collections, Calibre is indispensable.
Online converters like iformat.io are better for quick, occasional conversions. No installation required, no learning curve, and processing happens directly in your browser. For a detailed comparison, see our Calibre vs online converters guide.
Converting Kindle Books to EPUB
Converting Kindle books to EPUB allows you to read your purchases on non-Amazon devices. The process depends on whether the book has DRM. DRM-free Kindle books can be converted directly using Calibre or online tools. DRM-protected books require removing the DRM first, which exists in a legal gray area depending on your jurisdiction.
For a complete walkthrough of the process, including legal considerations, see our guide to converting Kindle books to EPUB.
Creating an eBook from Your Manuscript
If you are writing a book, your manuscript is probably in Word (DOCX) format. Converting a well-formatted DOCX to EPUB preserves chapter structure, headings, images, and basic formatting. The key is proper use of Word styles — headings should use Heading 1, Heading 2 styles rather than manually bolded text, because the converter uses these styles to create the table of contents and chapter navigation.
For detailed formatting guides, see our articles on preserving formatting in DOCX to EPUB conversion and converting your manuscript to EPUB for publishing.
Reading EPUB on Any Device
Apple devices read EPUB natively in the Books app. Android users can use Google Play Books (which accepts EPUB uploads) or dedicated readers like Moon+ Reader. On Windows and Mac, Calibre includes a built-in reader, and there are browser extensions that read EPUB files directly.
For Kindle devices, you can now send EPUB files using the Send to Kindle feature via email or the Kindle app. Amazon converts them to their format automatically. This means EPUB is now truly universal — it works on every major reading platform, either natively or through conversion.
Self-Publishing Format Requirements
Each publishing platform has specific format requirements. Amazon KDP accepts EPUB, DOCX, and KPF (Kindle Package Format). Apple Books requires EPUB. Kobo accepts EPUB. Google Play Books accepts EPUB and PDF. For maximum reach, create a well-formatted EPUB file — it is accepted by every major platform.
Pay attention to each platform's specific EPUB requirements. Amazon has particular expectations about cover image dimensions (2560 x 1600 pixels recommended). Apple requires EPUB 3 for enhanced features. All platforms validate your EPUB for errors before accepting it — tools like EPUBCheck can identify issues before you upload.
Key Takeaways
EPUB is the universal eBook standard supported by every platform except Amazon's store (though Kindle now accepts EPUB for personal documents). AZW3 is Amazon's proprietary format, locked to the Kindle ecosystem. PDF works as an eBook only for fixed-layout content like textbooks and comics.
For self-publishing, create a well-formatted EPUB and let each platform convert it as needed. For personal libraries, EPUB gives you the most flexibility across devices. Use Calibre for managing large collections and format conversions, and online tools for quick one-off conversions.
The eBook format wars have largely settled: EPUB won everywhere except Amazon. Focus on creating quality content in EPUB format, and you can reach readers on virtually every device and platform available.