My PDF files are deleted automatically

Hi guys. I have a mystery to be solved. For the past 1 month I noticed that PDF files randomly missing/automatically deleted when I either after I opened them or after a prompt message saying the file is missing/corrupted after I clicking on them. This has made my working life miserable as I keep on missing PDF files every day and some of them I don't have a copy. I tried looking in the Recycle Bin but it was not there. Did a full antivirus scan with Trend Micro, found nothing. Went on to the normal Google also could not find anything with regards to this issue. Appreciate your help in advance!