I had this problem and it was driving me to distraction and none of the fixes suggested worked for me.
I am not a techie but I found a way get round this problem as follows:
When you try to delete the file and you get the message that it cannot be removed because it is in use, (and you KNOW it is not), it offers you the option to try again. If you do try again with either a single or a double click it STILL will not delete.
HOWEVER, when I tried a quick TREBLE click then it DID remove the file. The timing of a treble click can be a little difficult so do not give up if it does not work first time. When I first started this I only got the three clicks right about one time in every
three however now I can remove any PDF file first time, every time.
Hope it works for you too. Good luck!!
Trevor.