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You bought an e-book from Amazon or published through Kindle Direct Publishing. The file is likely AZW3 — Amazon's modern e-book format, also known as KF8 (Kindle Format 8). It replaced the older MOBI format with support for HTML5 and CSS3, bringing Kindle e-books closer to the formatting capabilities of EPUB.
AZW3 is Amazon's proprietary container built on the EPUB/HTML foundation. Kindle purchases are typically DRM-protected AZW3 files that can only be read on Kindle devices and apps. Unprotected AZW3 files (from Kindle Direct Publishing or DRM-free publishers) can be converted to EPUB using Calibre. Under the hood, AZW3 supports complex layouts, embedded fonts, fixed-layout pages, and rich formatting — a significant upgrade from MOBI's limited capabilities.
Kindle devices and the Kindle app (every platform) read AZW3 natively. Calibre (free, all platforms) opens unprotected AZW3 files and converts between e-book formats. For publishing, Amazon accepts EPUB, DOCX, or KPF (Kindle Create output) — you don't need to create AZW3 directly.