Thanks to all for the hints. An update, if you have the patience...
I was about to 'Repair', when I tried a last chance: this time the pull-down at the bottom worked! It asked if to restore or sending (right-click on the file kept not working). Anyway I didn't restore the file because I didn't have at hand the removable drive it belonged to.
The following day I made the same. But no result! I did the Repair, restarted Windows (7): no result. Restarted once more: could restore two files but failed at the third. New Repair, new restart, and I finally managed to restore all the dozen PDFs.
Now for an off-topic: if I had to qualify Avast with a single word, it is UNPREDICTABLE. For about one year, some days Avast warned its Firewall was deactivated (though I had manually disabled Windows Firewall), some days there was no warning. Some days it says my network is vulnerable to Wannacry, some days not.
And don't mention SafeZone was part of the AIS I had bought, but after reinstalling Windows and Avast I learnt it is now a paid option.
And Avast knows my credit card for the automatic renewals, but if I want to add VPN or the like it asks for the number. Suspicious...
Enough. Is there somewhere inside or outside this Forum where Avast people will care to read the critics?
Thank you again!