There was a issue a few versions back on pdf files. Even if you allowed them in the control panel settings for attachments. I am not sure what version you are using but I am on 7.5.5 and it works fine for me now. I can't recall what the fix was at the moment. But it was in the bug forum. Probably under fixed bugs now.
Blue Man Group There is no such thing as stupid questions. Just stupid answers
I can't seem to find any reference to the pdf issues other than it was a issue at one point in time. So I am not sure what was done to fix the issue. Memory tells me it was you could upload a pdf but had problems opening the file afterwards. Maybe when Rick switches the search back to internal I can find it again.
Blue Man Group There is no such thing as stupid questions. Just stupid answers