We experienced this same problem.
If you display a SOP invoice to screen, then press the "SEND To" button in the window header, the moment you select the "Mail Recipient (PDF)" dropdown option, it creates a "SOP Blank Invoice Form.pdf" in the user's Temp folder.
In our case on Windows 7 the path was C:\Users\userID\Appdata\Local\Temp.
The PDF in this temp folder can be opened and reviewed as normal - BUT it remains in this folder, even once the e-mail has been sent. Even if the user logs-off the PC or shuts it down, the PDF still resides in this temp folder when they next log in.
When we generated the next e-mail invoice, we saw it start to create a new PDF (.ps file) briefly and then it replaced the existing SOP Blank Invoice Form.pdf with the new version. We could prove this by double-clicking and opening this file to see it was for the correct new invoice.
The PCs on which we experienced the problem were where there were permissions problems for the users with respect to the users ability to create and overwrite/replace files in the user's Temp folder. Once permissions were altered (or if you set the user to be Local Admin) the problem was resolved.