I never had this problem so obviously something needs tweaking. First thing to test is to leave the PDF out of the picture - check if the variables are received when performing a HTTP POST from another web page to your script. If this works, the next step would be to have your script do a phpinfo(); or print_r($_POST); and submit from your PDF. This in order to determine what really is sent from the client.
Now you should be able to pinpoint your problem.