Opening an EPUB or PDF in a reader app looks like a local operation. Often it is not. Conversion, text extraction, and “cloud sync” can all mean your file leaves the device, and the store listing rarely says so plainly.
Usually for a real engineering reason rather than a sinister one. Extracting text from a PDF is fiddly; doing it on a server is easier to build and easier to update. Sync across devices needs a copy somewhere. Neither is malicious. Both mean a copy of what you read exists off your device.
Whether that matters depends on what you read. For a public-domain novel, probably not. For a manuscript, a contract, a medical document or anything under NDA, it very much does.
It means the file is read from device storage, parsed on the device, and the extracted text stored on the device. No account, no upload, no sync. The cost is real and worth stating: nothing is backed up, and your progress does not follow you to another phone.
That is the trade Punctum makes. There is no account, no server, and no mechanism by which a book could leave the device — which also means we cannot restore anything for you.
Scanned PDFs — pages that are images of text rather than text — are not supported. Handling them requires optical character recognition, and doing that well on-device is a different piece of work. Rather than import an empty book and let you discover it later, the app tells you at import time.
Related: speed reading apps compared.