Hi Ravi
1
What folder are you copying those PDF files to, is that one of your user library folders?
2
Please test if this works:
Open Windows Defender Security Center
Go to Virus & threat protection - Virus & threat protection settings
Scroll down to Controlled folder access
Temporarily turn off Controlled Folder Access and check to see if you can open the PDF files
Please do not leave Controlled Folder Access turned off permanently