SUMMARY
Opened pdf file (with forms), did some edits, over-wrote file with another program, then tried to save the edited pdf on top of the original, overwritten file. I get the message:
The file '201802 foo.pdf' has been modified by another program, which means it can no longer be saved.
When I try to Save As, I get the message:
The file '201802 foo.pdf' has been modified by another program, which means it can no longer be saved.

STEPS TO REPRODUCE
1. 
2. 
3. 

OBSERVED RESULT
Cannot overwrite file, and cannot write the edited pdf file anywhere else.

EXPECTED RESULT
Even if some other program were modifying the file, (1) the save should overwrite the file after asking permission, or (2) I should be able to save the file under a different name.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION