EbookConverter wraps gryf/ebook-converter (a Calibre fork) to run natively on Android via Chaquopy.

Features:

Convert between 20+ ebook formats directly on your phone
Input: epub, mobi, azw3, azw4, docx, odt, fb2, html, htmlz, lrf, pdb, rtf, txt, djvu, chm, cbz, cbr
Output: epub, mobi, azw3, docx, fb2, html, htmlz, lrf, oeb, txt, txtz
Output profiles for Kindle, Kobo, Nook, iPad, and generic e-ink devices
Metadata editing (title, authors, publisher)
Font size and margin controls
Dark mode support
No internet required — all conversion runs locally

Limitations:

PDF input/output not supported (requires poppler and PyQt5, unavailable on Android)

Licensed under GPL-3.0.