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

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