@stefan I have recently given up on Paperpile and switched to Mendeley for precisely this reason. This is disappointing because I find Paperpile to be superior in just about every other way, but not having an easy way to sync up my local Bibtex file is a dealbreaker. I know it’s just a few clicks in the UI (and then a few more in downloading/copying to the right place), but having to do this over and over as I’m adding new citations for a document I’m working on is very tedious and just invalidates all the other ways Paperpile would otherwise be more convenient.
I’ll gladly switch back as soon as this feature is implemented, but given that users have been requesting this or similar solutions for years (Google Drive sync, Feb 2015, speeding up export, Dec 2017, public API, Dec 2015) I’m not currently holding my breath.