That's very strange. Try opening the files directly from your admin folder. They will still be there after the email is sent. This could rule out a problem with your email. If it is a problem with your email, you'll probably need to contact your server and see if they've done anything to their email system that could cause problems with attachments.

The other thing you could try is re-uploading the fpdf files. They are in admin/fpdf. You can copy them directly from the zip file you got for the mod or get them at fpdf.org