A favorite (though a website and not an app):
https://en.m.wikipedia.org/wiki/Sci-Hub
For getting high quality PDFs of research papers, technical books, industry standards, etc.
The EFF profiled the founder not too long ago:
https://www.eff.org/awards/effawards/2023#award-1
I also use CamScanner (network permission revoked, no Google Play services installed) to scan physical print media, handwritten notes, etc because of the superior algorithm for removing shadows / filtering to black&white and nice PDF handling