Kindle supports a variety of file formats for eBooks, including:

• Amazon KF8 ebook (.azw3)
• Amazon Print Replica ebook (.azw4)
• EPUB ebook (.epub)
• Unprotected PRC ebook (.prc)
• PDF document (.pdf)
• Microsoft Word document (.doc, .docx)
• Rich Text Format document (.rtf)
• Plain Text document

Kindle devices and apps are designed to use Amazons e-book formats, such as AZW and MOBI. However, Amazon has added support for EPUB files, which can be added to a Kindle Library and automatically converted to KF8 files. Existing MOBI and AZW files in a Kindle Library are converted to the KF8 (.azw3) format, but adding new MOBI and AZW ebooks is no longer supported by Send to Kindle. If you have MOBI or AZW ebook files that you want to add to your Kindle, you’ll need to use Calibre to convert them to EPUB format and then use Send to Kindle to get them into your Amazon Digital Content library.

In summary, Kindle supports a variety of file formats, including Amazons proprietary formats, EPUB, and other common document formats such as PDF and Microsoft Word.