There is a limit on the size of the uploads in the admin under attachments. There is also a user upload limit amount also in the user admin. Maybe you don't have these settings set to high enough values?
I have finally managed to find out what the issue was on my hosting. The following is the note from my support guys as to my issue. I suppose this might be common as a possible cause in the failing of large pdf uploads when all admin sections are set up correctly.
"The mysql start up paramemter was not high enough to accomodate the upload, and that has been corrected."
Hope this assists others with possible issues that are the same