This is an environmental issue and is commonly caused by any of the following:
Temporary network failure when the PDFs were being generated or emails sent.
The account used by the safe mail service lost permissions to the location where the PDFs are saved.
The folder containing the PDFs contains a very large number of files, causing performance issues.
Any other underlying issue affecting performance on the server sending the emails, the DB server, Pay & Bill application server or file sever where the PDFs are saved.