Kindle, EPUB and the format alphabet
The first time a store asks you to upload your ebook, you meet a wall of abbreviations: EPUB, MOBI, AZW, KPF, KFX, PDF. It reads like a decision. It mostly is not.
Here is the short answer, and the rest of this guide is why it is true: make one valid EPUB and upload that everywhere.
EPUB is the standard, and nearly every store takes it
EPUB is the open ebook format — an industry specification, not one company's product. Apple, Kobo, Barnes & Noble, Google, the library distributors and the aggregators all take it. So does Amazon.
That last one surprises people who remember older advice. Today you generally upload an EPUB to Amazon like anywhere else, and Amazon converts it on their side into whatever internal format their apps and devices currently use. You do not make that file. You do not see it. You are not expected to manage it.
So the format question, for a normal book of words in a row, has one answer. What varies between stores is the metadata, the pricing, the categories and the terms — not the file.
What happened to MOBI
MOBI was Amazon's old format, and for years the advice was to keep a MOBI beside your EPUB. That advice is retired along with the format. MOBI is gone, and Amazon's internal formats have changed more than once since, which is precisely the point: an internal format is theirs to change.
Every time it changed, authors who had carefully maintained a store-specific file found the file was now the wrong one. Authors who kept a clean EPUB found nothing had happened to them at all.
That is the durable lesson. Own the standard file. Let the store own its own conversion. You are not responsible for the shape their device wants this year.
In the Interior Formatter. It builds an EPUB 3 from your .docx — the file you upload to every store, Amazon included — and there is no second, store-specific version to keep in sync. Start a book → · about the Interior Formatter
Why one file per store goes wrong
The tempting alternative is a folder with book-amazon.epub, book-kobo.epub, book-apple.epub, each hand-tweaked for something you noticed once.
What happens next is always the same. You find a typo on page 60. You fix it in one file. Six months later a reader on a different store reports the same typo, because that file never got the fix. Then you cannot remember which of the four is current, so you open all of them and compare, and the comparison takes longer than the fix.
Files drift the moment there is more than one of them. The version you tested is not the version you shipped, and you will not notice until a reader tells you.
If a specific store genuinely needs something different — a different ISBN in the metadata, say — change it at upload time, in their form, and leave the master file alone. Regenerate from the manuscript rather than editing an export. An export you hand-edit is a file with no source.
The other formats, and what each is actually for
- PDF. Fixed pages. Excellent for print, where you decide every line break and every margin, and reasonable for a document someone will print at their desk. A poor ebook: it cannot reflow, so a reader on a phone pinches and pans instead of reading. Some stores accept it. That does not make it a good choice. See EPUB and print PDF are two different books.
- A plain reading file for reviewers. Early readers and reviewers usually just need something they can open. An EPUB works; so does a PDF if you would rather send something that cannot be reflowed or easily edited. This is a delivery decision, not a format your book is made in — see Getting early reviews without breaking a rule.
- Fixed-layout EPUB. A variety of EPUB where the page is frozen, for books where the page is the design: picture books, comics, graphic novels, some cookbooks. It is the wrong tool for a novel, because it takes away the type size control readers depend on.
- .docx. Your manuscript, not your ebook. It is the source everything else is built from. Keep it clean and keep it current.
What a store's preview tool does and does not tell you
Every store has an in-browser preview. It is useful for one thing: seeing roughly how your file lands in that store's own rendering.
It does not tell you whether your file is sound. A preview can look perfectly acceptable while the file underneath has broken navigation, missing structure, or errors that will trip a different reading system entirely. The preview is one interpreter's reading of your file, and a forgiving one.
Two checks are worth more than all the previews put together.
Run a validator. epubcheck is the standard tool, and it reports structural problems directly instead of quietly rendering around them. Zero errors and zero warnings is an achievable bar, not an aspiration. What makes an EPUB pass validation covers what it looks for.
Read it on a real device or two. A phone and an e-reader, or a phone and a tablet. Change the type size. Turn on the largest setting and see what happens to your chapter openings. Ten minutes of this finds things no preview shows you.
The principle
One master manuscript. One valid EPUB. Many stores.
The manuscript is where you fix things. The EPUB is generated from it, never edited after the fact. Each store takes that same file and does whatever it does — converts it, renders it, wraps it in their own furniture — and none of that is your problem to manage, as long as what you handed them was correct.
Your cover is the one thing that genuinely is a separate file at a separate size, because stores show it as a thumbnail rather than reading it. Ebook cover size covers that.
In the Interior Formatter. The EPUB it produces validates with zero errors and zero warnings under epubcheck, with accessibility metadata and proper chapter navigation already in place — so the file you send to every store is the same sound file. Start a book → · about the Interior Formatter
Make yours
Everything here is built into the tools, and they are free to use: name your book, bring your manuscript or your artwork, and work as long as you like. You only pay when you want the files without the preview watermark.
Next: What makes an EPUB pass validation · More: EPUB and print PDF are two different books · Ebook cover size · Accessible ebooks