Ebook File Formats Explained: EPUB, MOBI, KPF, PDF and More

Ebook file formats used to be a genuine headache, with different stores demanding different files. That has simplified a great deal. Today, one well-made EPUB covers most of the market, and the main stores convert it into whatever their devices use. Knowing what each format does still helps you avoid common upload problems and choose the right layout for your book.

The main ebook formats at a glance

EPUB: the format to master

EPUB is an open standard, now maintained under the W3C, and it is essentially a packaged website: HTML text, CSS styling, images, a table of contents and metadata, zipped into a single file. Because it is open, any reading app can support it.

• Reflowable by default. Text adapts to the reader's screen, font and size settings.
• Navigable. A proper EPUB has a built-in table of contents readers can jump through.
• Accessible. EPUB 3 supports semantic structure and image descriptions for screen readers.
• Widely accepted. One EPUB can go to almost every store, including BooksOasis, where readers open it in Oasis Reader.

The quality of an EPUB depends on how it was made. Clean, styles-based formatting produces a clean file; a document full of manual spaces, tabs and hard returns produces a messy one. Our guide to formatting an ebook from Word walks through the process.

What happened to MOBI?

MOBI was the standard Kindle format for years. In 2021, Amazon stopped accepting MOBI files for new reflowable ebooks on KDP, recommending EPUB, DOCX or KPF instead, and in 2022 it began letting readers send EPUB files to their Kindles. Amazon converts uploads into its own formats (such as KFX) automatically.

In practice: you no longer need to create MOBI files. Upload EPUB to KDP like everywhere else.

KPF and Kindle Create

Kindle Create is Amazon's free formatting tool. It exports a KPF file that only works on KDP. That is fine if you only sell on Amazon, but if you also publish on Apple, Kobo, Google, BooksOasis or through a distributor, you will need a separate EPUB anyway. Many authors therefore prefer a tool that exports EPUB and print PDF from the same manuscript. See the best book formatting tools for the options.

PDF: right for print, wrong for most ebooks

PDF preserves every page exactly. That is perfect for print books, where the interior must match a trim size, and useful for workbooks, journals and planners that readers print. But on a phone, a PDF of a novel means pinching and zooming page by page.

Reflowable vs fixed layout

Reflowable ebooks suit almost any book that is mainly text. Readers control font, size and spacing, and the text flows to fit. You give up precise page control, so avoid designs that depend on exact positioning.

Fixed-layout ebooks keep each page or spread exactly as designed, like a PDF but in EPUB form. They suit picture books, comics, art books and some cookbooks. They are harder to produce, less comfortable on small phones, and not supported equally by every store, so use them only when the design truly requires it.

Metadata inside the file

An EPUB carries metadata: title, author, language, identifier (often an ISBN), publisher and description. Stores mainly use the details you type into their dashboards, but correct internal metadata avoids mismatches and helps reading apps display your book properly. Check the title and author fields match exactly what you enter at upload.

Accessibility

Accessible ebooks are increasingly expected, and in the European Union the European Accessibility Act has applied to many ebooks sold there since June 2025. Good practice that costs little:

• Use real headings for chapters rather than bold text.
• Add alternative text descriptions to meaningful images.
• Never use images of text for body content.
• Keep a logical reading order and a working table of contents.
• Set the book's language in the metadata.

For a deeper checklist, LiberScript's ebook accessibility guide covers structure, image descriptions and testing.

Checking your file before upload

1. Validate it. Run your EPUB through EPUBCheck, the official validator. Many stores run it too and reject files with errors.
2. Preview on a phone and a larger screen, in at least two reading apps.
3. Test the table of contents: every entry should jump to the right place.
4. Check images for sharpness and size; oversized images bloat the file.
5. Try dark mode and a large font to catch hard-coded colours and layouts that break.
6. Read the first and last chapters closely, where formatting errors often hide.

Which formats does each store want?

Store requirements change, so confirm on each platform's help pages before you publish.

How to create an EPUB

You do not need to write HTML to make a good EPUB. The main routes:

Whichever route you choose, the same rule applies: format with styles (Heading 1 for chapters, a body text style for paragraphs), not manual spacing. Our comparison of book formatting tools covers the main options in detail.

Images, fonts and file size

• Images: use JPEG for photographs and PNG for line art or images with text. Size them for screens, not print; very large images bloat the file, and some stores charge delivery fees based on file size.
• Covers: the cover image is usually uploaded separately as well; see ebook cover size for dimensions.
• Fonts: most reading apps let readers override fonts. Embed fonts only when they matter (for example for headings or special characters), and only if your font licence allows embedding.
• Special elements: tables, footnotes and poetry need extra care, because reflowable layouts can break them. Test them on a small screen.

Do you need separate files for each store?

Usually not. One EPUB works on almost every store. You may want small variations, such as back-matter links pointing readers to each store's version of your next book, but many authors simply link to a universal book page or their author storefront instead, so one file serves everywhere. If you sell direct, the same EPUB you upload to BooksOasis is the file readers receive.

Audiobooks and print: other formats to know

Print files are a separate job from your ebook: they need a trim size, margins, page numbers and a cover with a spine. Our guides to trim sizes and print margins cover the essentials.

Common format mistakes

• Uploading a PDF as the main ebook for a text-based book
• Creating only a KPF file, then needing to reformat for every other store
• Fixed layout for a novel, which frustrates phone readers
• Skipping validation and getting rejected at upload
• Huge uncompressed images that make the file slow to download
• Formatting with spaces and tabs, which breaks reflow

In short

Make one clean, validated EPUB for your ebook and a print-ready PDF for your paperback, and you are covered almost everywhere. Use fixed layout only when the design demands it, and leave Kindle's internal formats to Amazon.

Curious how readers find books? Browse our reading guides on the BooksOasis store.