A thought r.e. sidestepping the problems where the android app does not save annotations on samsung tablets:
A possible alternate approach would be to install the google drive app, open the Gdrive paperpile folder, open the particular file with a pdf editor, make the desired annotations, close the app (or do whatever to insure that the file is saved).
This would have the advantage that we could use any app (with its full features), however I do not know how it would work offline. At the very least I guess one would need to open the file in the app before going offline to ensure that it is cached.
Do you think this should work / is anyone using this?
I briefly tried it on a test file (using the Xodo editor). It successfully saved the annotation, however it does not seem to record the chosen font size (the annotation is huge).