I checked our RIS export and the relative path to PDF attachments (as they are stored in Google Drive: All Papers/[A-Z]/filename.pdf) is correctly written to the L1 field in the RIS file. As Stefan wrote above in (1) we can’t just write the absolute path to the PDF file there, as we do not know to what exact location you are syncing your Google Drive content to.
But the problem can be solved by a simple find & replace in a text editor. Just replace
L1 - All
with the actual location on your hard drive, e.g.
L1 - file:///home/agruber/test/All
In the example I tired, the L1 line then looked as follows:
L1 - file:///home/agruber/test/All Papers/K/Köhn and Hüttelmaier 2016 - Non-coding RNAs, the cutting edge of histone messages.pdf
I imported the RIS file in Zotero and the PDF file was correctly attached to the record.