We are having problems where open PDFs and particularly those that are in some sort of edit mode over the network refuse to save or report not available or locked with the slightest bump in the network. For instance shifting from wired to wireless, a VPN bump, anything, including if the computer goes to sleep/standby.
Word, Excel, and any other such all things handle the transitions fine. But, if PDF Xchange senses the slightest/briefest disconnect it refuses to make the connection again, usually then losing work in the process. And, it won't let you do a save as even to the network location either over top of the original name or to a new name.
For our test/control case:
Windows 10 laptop, fully up to date
PDF-Xchange v6, fully up to date
I see there were some old posts about this issue back around 2013 and older.