Hi Eric,
first of all thanks a lot for your support.
I tried changing account and starting in "safe mode". When I changed account I opened "whatsapp web" (as telegram web) and after I tried to download a pdf file. I managed to open it. Then I returned in my account but when I tried i couldn't open the same file.
After I started operating system in "safe mode" but also in this case I obtained the same result.
With another account I can see pdf, with my account (and I am the administrator) I couldn't see it.
A thing I discovered is the fact that with my account, when I try to download a file, it open a new tab and reply with an error string. With other account the file downloading go to the “download icon” starting the blue download line (and when I enter in download folder I can open pdf file correctly)
I think it depends on some setting in my account but I didn't understand how I can risolve my problem.
Thanks for your patience