Suddenly we started getting the security prompt when opening PDF files. You can click Yes, but it unblocks only that single file. This can be eliminated via Group Policy:
Via GPEDIT.MSC, navigate to:
Local Computer Policy
User Configuration
Administrative Templates
Windows Components
Attachment Manager
Enable the “Inclusion list for low file types” policy and add the file type(s) with a leading period and trailing semicolon like this:
.pdf;.gif;