can not open the second pdf document sent by a form via email

I created a form since 2010 with a button that transmits the form by email. The customer must add another pdf document to the email and forward it. On 1500 emails / days, +/- 20 emails that I can not open their pdf document. These client transmit for years and it worked. I have this problem since January 8th. Customers have different Adobe version, OS, Antivirus and e-mail editor. They do not have a central point. Do you have an idea of soulution?