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EPUB, MOBI, and the formats bookstores actually want

Three file extensions do most of the work in publishing, and one of them is finished as a format to produce. What each format is for, what Amazon takes now, and why a fixed-layout book is a different product.

The Nueel team9 min readUpdated 5 September 2026

The file-format question in publishing is smaller than it looks, because the industry has consolidated around one answer: make an EPUB. Everything else is either an internal conversion someone else performs, a print format, or history. But the history still gets asked about, and the one real distinction — reflowable against fixed layout — is genuinely important.

EPUB: the format the industry agreed on

An EPUB is a ZIP package of HTML files, styles and images, with metadata and a navigation document — in effect, a small website describing a book, wrapped to a standard. That construction is the source of both its strengths and its quirks: the text is real text that reflows to any screen and any type size the reader chooses, and everything the book contains is inspectable and convertible.

It is the format every major retailer accepts directly — Apple Books, Kobo, Google Play, Barnes & Noble — and the one Amazon converts from. It is also what libraries lend through their apps. One file serves all of them, provided it is built well — and it is still tested per platform, because each retailer validates and renders the file its own way, and not every reading system supports every EPUB 3 feature. The errors that get one rejected are covered in why your EPUB was rejected.

MOBI: the format that lost

MOBI was Kindle’s native format for most of two decades, and self-publishers built their mental model around it: “convert to MOBI for Amazon”. That model is out of date.

Amazon stopped accepting new MOBI uploads for reflowable ebooks on 1 August 2021, and for fixed-layout ebooks in March 2025; the EPUB is what it takes now, converted on Amazon’s side. Send to Kindle accepts EPUB, alongside PDF and Word files. The format itself froze years before that — it never gained the features modern EPUBs carry, which is why it lost.

Two residues keep the word alive. Files already published as MOBI keep working, and a few input routes still accept it: Kobo Writing Life takes MOBI uploads and converts them to its own EPUB. And Kindle Previewer can still export a MOBI file for sideloading onto older Kindle devices that never received firmware updates. Neither is a reason to produce one: any guide still telling you to convert your EPUB to MOBI is describing a world that has ended.

KPF is Kindle Create’s package format — a convenience route Amazon still accepts. It is not required, and choosing it binds the book to one tool; the EPUB is the portable choice.

PDF: the format of fixed pages

A print-ready PDF is a different object from an ebook: fixed pages, embedded fonts, exact dimensions — the requirements are the print-ready checklist. It goes to a press or a print-on-demand service.

PDF is not, however, only a print format. It is a fixed-page document format — with selectable, searchable text when built properly, not a photograph of a page — and the stores know it. Google Play Books sells PDF ebooks and actually recommends submitting an EPUB and a PDF together, so readers can choose between reflowable text and the original pages. KDP accepts PDF ebook manuscripts in certain languages, and Send to Kindle takes PDFs.

The honest rule is about the reading experience, not about permission: a PDF’s pages do not reflow, so on a phone the page is a postage stamp and the reader’s type-size settings do nothing. For a novel or ordinary nonfiction, EPUB is the better reading format and PDF is what the print edition needs. A few books — art, complex reference — genuinely are page-objects, and for them the fixed-layout EPUB exists, with capabilities a PDF does not have.

The one real distinction: reflowable vs fixed layout

Everything above sorts into two kinds of book.

Reflowable — the default for anything read as text. No fixed pages; the reading system paginates dynamically, and the reader’s device and type choice decide the layout. This is where everything an ebook does not control applies, and it is what “EPUB” means in most conversations.

Fixed-layout EPUB — each page rendered as designed, pre-paginated rather than flowing. For picture books, comics, and heavily designed cookbooks, it is the correct product: the composition is the content, and reflowing it would destroy it. The costs are real: readers generally cannot change the type size. And that makes accessibility harder, not optional — the book still needs a logical reading order, real structure and navigation, text alternatives for meaningful images and accessible metadata, on top of design choices about contrast and legibility. A fixed-layout EPUB built without those can be partially or wholly inaccessible, which the standard itself warns about.

The mistake to avoid is choosing fixed layout because the book looks designed that way in your editor. A novel with a pretty chapter opening is still a reflowable book; the opening is styled content at the start of the chapter, and the page boundaries remain the reading system’s to draw. Fixed layout is for books where every page is composition.

What you actually make

The working recipe for a typical book:

  • One EPUB, reflowable, well-built and validated — it uploads to every retailer and every library platform, tested per store.
  • One print-ready PDF, at the chosen trim, with fonts embedded — it uploads to KDP, IngramSpark or any printer.
  • The cover assets each destination needs — an image for the ebook listing, a full-wrap cover file (back, spine and front) for print, per setting up a cover file.

Two book files and their covers, made from one book. Every other extension in the ecosystem is derived from these: AZW and KFX are Kindle’s delivery formats, produced on Amazon’s side from the file you upload. MOBI is the one that is not downstream of anything — it is the legacy format itself, kept alive only for old devices, and nothing new should be made in it.

The formats settled while nobody was announcing it: EPUB for reading, PDF for printing, everything else generated downstream. The craft has moved to making the EPUB genuinely good — structure, navigation, notes and images that survive every screen.

Nueel exports both from the same book: a print-ready PDF from the paginated design and a reflowable EPUB from the same text, so the two artefacts agree because they were never two projects.

Sources

Common questions

What file do I upload to Amazon KDP now?
For a professionally prepared ebook, EPUB is the standard portable choice. KDP accepts EPUB directly and converts it into the Kindle formats delivered to readers. KPF from Kindle Create, Word, HTML, RTF and text files are also accepted, and PDF in certain languages. MOBI is no longer accepted for new or updated ebooks.
Is MOBI dead?
As a format worth producing today, effectively yes. Amazon stopped accepting MOBI for reflowable ebooks in August 2021 and for fixed-layout ebooks in March 2025, and Send to Kindle has moved to EPUB. Some platforms still accept MOBI as input and convert it — Kobo Writing Life does — and Kindle Previewer can still export it for sideloading onto older Kindle devices. But there is little reason to create a new MOBI file.
What is the difference between a reflowable and a fixed-layout EPUB?
A reflowable EPUB has no fixed pages: the reading system paginates the flowing text to fit the screen and the chosen type size, which is what novels and most nonfiction want. A fixed-layout EPUB pins each page's design in place — picture books, comics, cookbooks where the composition is the content — at the cost of the reader's typography controls. They are different products built from different sources, and a fixed-layout book still needs its own accessibility work.
Why not just upload a PDF?
For a novel or ordinary nonfiction, because a PDF's fixed pages do not reflow: on a phone the page is a postage stamp, and the reader's type-size settings do nothing. PDF is not forbidden as an ebook — Google Play Books sells PDF ebooks and can offer one alongside an EPUB, KDP accepts PDF manuscripts in some languages, and Send to Kindle takes PDFs. But for text-heavy books, EPUB remains the better reading format, and PDF is the format a print edition actually needs.

Nueel is a web app that turns a finished manuscript into a typeset book, in your browser: import your Word file, choose a design, export a print-ready PDF and an EPUB. See how it works.