Every Kindle-Supported File Format Explained

A Kindle does not open every ebook file the way a computer opens a text document. Amazon built its ecosystem around a small family of proprietary formats, and it accepts a handful of common document types that it converts on the fly. Understanding this split (native formats versus accepted-and-converted formats) is the key to knowing why some files load instantly while others need a quick conversion first.

Native Kindle formats are the ones the device stores and renders directly, giving you full control over font size, margins, and reflowable text. Accepted formats are files you can send to Amazon, which then packages them into a Kindle-friendly version delivered back to your library. Both paths end with a readable book, but the native route preserves the most features.

AZW is the original Kindle format, based on the older Mobipocket structure. It handles basic reflowable text well but lacks the richer styling options of newer files. Most books bought in the early Kindle years used AZW, and modern devices still read it without any trouble.

AZW3, also called KF8, expanded what a Kindle page could do. It added support for embedded fonts, better tables, nested styling, and more precise layouts, making it the go-to choice for anyone sideloading a well-formatted book. If you convert an EPUB with a tool like Calibre, AZW3 is usually the best target format for older and mid-generation devices.

KFX is Amazon's most current format and powers advanced typography features such as improved kerning, hyphenation, and the Bookerly font enhancements. It is generated by Amazon for content delivered through your account, so you rarely create KFX files yourself, but it is what your newest Kindle uses behind the scenes for the sharpest reading experience.

Beyond ebooks, a Kindle library can hold personal documents. Amazon accepts Microsoft Word files (DOC and DOCX), plain text (TXT), rich text (RTF), and HTML pages, converting each into a readable Kindle format when you send it through Send to Kindle or your personal email address. This makes the device handy for manuscripts, work reports, and long articles.

PDF sits in a special category. Kindles can display PDF files directly, which keeps the exact page layout intact, but that fixed layout means text does not reflow to fit smaller screens. For a better reading flow, you can ask Amazon to convert the PDF during transfer, though complex layouts with columns or heavy graphics may not translate cleanly.

EPUB is the most widely used open ebook standard, yet Kindles have never opened EPUB files natively. For years this meant converting every EPUB before reading it. That friction is largely gone now, because Amazon updated Send to Kindle to accept EPUB files and convert them into a Kindle format automatically.

If you prefer to convert EPUB yourself, free software such as Calibre turns an EPUB into AZW3 or a Kindle-ready file in a few clicks, letting you check formatting before it reaches your device. Either way, an EPUB from a library, a store, or your own writing can end up on your Kindle without much effort, so the format is no longer the barrier it once was.

MOBI was long the informal standard for sideloaded Kindle books, and countless older files still use it. Devices can generally still read MOBI stored on them, but Amazon removed MOBI support from Send to Kindle in late 2022, so you can no longer email a MOBI file to your library. If you own MOBI books, convert them to EPUB or AZW3 to keep them future-proof.

PRC is a close relative of MOBI and behaves the same way, working on the device but no longer welcome through the transfer tools. Treating MOBI and PRC as legacy formats, and converting the ones you want to keep, spares you from surprises when you upgrade to a newer Kindle.

Kindles also handle several image types, including JPEG, PNG, GIF, and BMP, which matters for comics, manga, and illustrated pages. These images can be embedded inside ebooks or sent as personal content, and they display according to the screen size and resolution of your specific device.

For comic-heavy reading, formats built around image collections work best when they are packaged into a Kindle file first. Keeping images at a reasonable resolution helps them look crisp without bloating file size, especially on the higher-resolution screens found in recent Kindle models.