Decide what privacy means for your books
For a personal library, privacy may mean that your files never leave the phone. It may also mean reading history, notes and highlights stay local. A reader can support offline books but still send optional diagnostics or synchronize progress through an account. Read its privacy policy and check what settings are enabled.
Test without a login
Import a file you own into the reader, disconnect from the network and reopen it. Try bookmarking, searching and changing appearance. If ordinary local reading continues, you are less dependent on a service staying online. This does not prove the app sends no data when connected, but it helps separate local features from cloud ones.
Check permissions carefully
A system file picker is usually enough for importing one book. If an app asks for broad storage, contacts or unrelated permissions, look for an explanation tied to an actual feature. Some Android versions display different permission prompts, so compare what the app requests with the work you want it to do.
Think about backups before deleting anything
An offline-first library may not automatically back up your highlights, notebooks or reading positions. Ask whether you can export them. Keep the source EPUB/PDF files in a second secure location. If an app offers cloud sync, find out whether it stores whole books, metadata or only progress.
For Readream, check the published privacy information for the Basic or Pro edition you use. Don't assume that two editions have identical feature behavior; review the details and test with a harmless sample file first.
If you want to read local files on Android, consider Readream Basic and review its privacy policy before installing. Downloading catalog books or voice previews still makes requests to the relevant online providers.