How to release it without ending the program
Acrobat often refuses the request to close (error 351). Close the PDF window yourself; if that does not work, it has to be ended.
If that still does not work
- Close the program and delete
- Unlock and delete
- Move to Recycle Bin
- Delete after restart
Get LetGoFile and drop the file on it — it names what is holding the file, PID and all.