Which file formats a Kindle actually accepts

Amazon has changed the rules twice in the last few years and most of the advice online still describes the old ones. Here’s where it stands now.

What you can send today

Per Amazon’s own Send to Kindle documentation, the web uploader, the apps and the email address accept:

• EPUB (.epub)
• PDF (.pdf)
• Plain text and rich text (.txt, .rtf)
• HTML (.htm, .html)
• Images (.jpg, .jpeg, .png, .gif, .bmp)
• Microsoft Word (.doc, .docx), with a caveat: the Send to Kindle page still lists Word, but the main help page dropped it in 2024. If a Word file won’t go, save it as PDF or EPUB.

Anything else has to be converted before it will go across. The web uploader takes files up to 200 MB. Email is capped at 50 MB per message, across up to 25 attachments.

MOBI is finished

Send to Kindle stopped accepting MOBI (and the older AZW and PRC) on 20 December 2023. Amazon first announced the change in 2022 for “late 2022”, and their Send to Kindle help page moved the date twice before support was wound down from 1 November 2023. It catches people out, because MOBI was the Kindle format for fifteen years and half the converters online still push it at you as the obvious pick.

Anything already in your library keeps working, and a MOBI copied over USB still opens. But convert a PDF to MOBI today, try to send it across, and it bounces. That’s far and away the most common reason a conversion “doesn’t work”. The file is fine. Amazon just won’t take it any more.

EPUB is the format to use now. Amazon converts it to their own internal format on receipt, which is exactly what they used to do with MOBI.

What a Kindle opens by itself, over USB

Sending and sideloading are different lists. Amazon’s Paperwhite spec page puts it plainly: AZW3, AZW, TXT, PDF, unprotected MOBI and PRC “natively”, and EPUB, Word, HTML, RTF and images only “through conversion”, which means Send to Kindle.

So an EPUB copied over USB won’t open. If you want a book on a Kindle without it going through Amazon, convert the EPUB to AZW3 in Calibre first and copy that across.

AZW3, KFX and the formats you never see

You will run into these names when reading about Kindle internals:

• AZW3 (KF8) — Amazon’s older modern format. Kindles still read it natively, which is why it’s the one to sideload.
• KFX — the current one, with better typography and layout. Amazon makes it on their side; it isn’t something you produce or sideload yourself.

If you’re using Send to Kindle you don’t need either. Send EPUB and let Amazon handle it. Their converter knows their hardware better than anything you’ll run at home.

Why send EPUB rather than the PDF itself

A Kindle will happily display a PDF, so if that’s all you’ve tried you might wonder what the fuss is about. The trouble is the fixed page. On a six-inch screen it gets shrunk until the type is too small to read, leaving you pinching and panning, or turning the thing sideways to read two paragraphs at a time.

An EPUB reflows. The text rearranges itself around whatever screen and font size you’ve picked, so the Kindle can show it the way it shows a book you bought from them. Which is the thing the device is actually good at. Reflowable layout is what the EPUB 3 specification is built around, and it is why every e-reader made in the last decade reads the format.

Converting to EPUB first is what buys you that, and it’s why the quality of the conversion matters. It has to rebuild paragraphs and chapters the PDF never recorded in the first place.

Sending a PDF anyway

Sometimes you should. If the layout is the content, reflowing it wrecks it. Sheet music, knitting patterns, circuit diagrams, annotated maps: send those as PDFs and read them in landscape.

Anything that’s basically prose, convert it.