Cannot open or delete files under Documents folder

I cannot open certain kinds of files under Documents folder, like excel or word (but not just Microsoft office files), and get a prompt says "The file “xxx.pdf” couldn’t be opened." or the corresponding software starts but just not opening the file.

And I cannot delete any file under Documents folder.

Plus, I checked the "Get Info" menu, in the "Sharing & Permissions" section, it says "You can read and write".

Other folders under my User's path seems to work fine.

Can't understand why, does it have something to do with Catalina's new security restrictions? If so, how can I be granted these operations?

PS, I updated to macOS Catalina a while ago, but I believe I have been able to open excel files after the update for several times.