CamScanner is a free Android app that takes photos of documents, automatically crops the photo, enhances the photo, saves it, categorizes it, tags it, converts it to PDF, and uploads it to your cloud. And CamScanner does it well.
The quality is as good as (or better than) a normal scanner. Here is a sample page I captured from a book: Examplepage .
Try it out: https://www.camscanner.com/ru?208390158tw