After reading this thread, I would like to offer the following hopefully helpful suggestions.
Since stored PDF files reside in a folder that is accessible from the mobile Google Drive app can't you just browse that directory tree to read articles in your Paperpile collection?
Since you can make annotations in various readers when opening from Paperpile, I assume this does not corrupt the Paperpile database, although maybe there is some reindexing that goes on in the background when opening a PDF from within Paperpile?
There is a feature of the F1000 service, from another research tool provider, that allows you to select a monitored folder for adding new references. If Paperpile had a similar sub-folder for scanning new references, it would simplify adding them from any device. Tags and other Paperpile database specific annotations would need to be added afterwards from within Paperpile.
Haven't experimented yet, since I am not inclined to risk corrupting my Paperpile database until I hear back from this forum.