I just had the same issue oddly enough and was able to resolve it. Check the the last post in this thread: http://xenforo.com/community/threads/unable-to-attach-any-files.25584/
both sites are on the same server.. And looked at the allowed attachments and both are the same..Presumably pdf is an allowed attachment type and the maximum attachment size is greater than the file?
Are both sites on the same server?
That worked...Does it happen with all attachments or only pdfs? Or only that one pdf?
I have seen this caused by using the flash uploader on a forum that is behind htaccess:
Admin CP -> Home -> Options -> Attachments -> Use Flash Uploader
The flash uploader simply doesn't work behind htaccess.