Amazon has largely moved to the AZW3 (Kindle Format 8) for newer books.

The Amazon Kindle App (available for PC, Mac, iOS, and Android), Mobipocket Reader, or Stanza.

The Complete Guide to MOBI Files: Creation, Conversion, and Downloads

As of early 2025, Amazon no longer accepts MOBI for new ebook submissions on Kindle Direct Publishing (KDP), preferring EPUB or KPF instead. 2. How to Create or Convert MOBI Files

MOBI (short for Mobipocket) is a reflowable ebook format. Unlike PDFs, which have a fixed layout, MOBI text adjusts automatically to fit the screen size and orientation of your device.

For years, it was the primary format for Kindle devices.