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Orion Viewer vs Moon+ Reader: catalog facts
Orion Viewer	Moon+ Reader
Free Free and open source under the GPL.	Freemium Free version available, with a paid Pro version.
Android	Android
Opens DjVu as well as PDF, XPS, CBZ and TIFF Configurable tap zones and key bindings for page turning	Very wide format support including Markdown and CHM Reads books inside ZIP and RAR archives and OPDS catalogs
Viewer only, with no PDF editing Interface is functional rather than polished	Android only Some features require the paid Pro version
Downloadable app	Downloadable app
Open source	Closed source
License: GPL-3.0-or-later	License not stated
No account needed	Not stated if an account is needed
Works offline	Not stated if it works offline
Official website Full listing and sources Checked October 2, 2026	Official website Full listing and sources Checked September 23, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.