"[Errno 1] Operation not permitted" on particular files causes backup to fail

Dear all,

I am backing up my home folder to an external usb-drive and all of a
sudden, the backup started failing due to the following error:

ListError: 'Documents/somefile1.pdf' [Errno 1] Operation not permitted:
 b'/home/user/Documents/somefile1.pdf'
ListError: 'Documents/somefile2.pdf' [Errno 1] Operation not permitted:
 b'/home/user/Documents/somefile2.pdf'
Previous backup seems to have failed, regressing destination now.

I found out that they were pdf-files that I cannot open myself in linux,
so I just removed them and started re-running the backup. However, I
don't want my backup to fail every time someone sends me some
'security-enabled' Adobe pdf file, so I wondered if there is a way to
make rdiff-backup continue the backup on such errors and at the end
provide a list of files that were not backed up. Thanks already for any
tips!