This problem is STILL not fixed in 188.8.131.5292!
Going back to 6.7.5 (much grumbling as there are bugs in 6.7.5 that occasionally and irrevocably corrupt and overwrite PDFs). Only remedy is to dig back through backups and recover the one PDF. What a WOFTAM!
I just raised this ticket:
PDF drag and drop is BROKEN from 6.8 through to 6.17 - PLEASE FIX
Let's see if my firm but polite request will be escalated to the developers.
I asked for a feature in the EN options menu that allows us to choose between old style (6.7 and before) or new style (6.8) PDF presentation modes any time we want to switch.