Frames, PDF and links problem

Here is the problem I am facing now. I have a site, with a sort-of shopping cart but heavily modified for the job it does. The site uses 2 frames: 1 for the cart and 1 for the content. The customer has Acrobat PDF files that they want links in to add things to the 'cart'. If PDF files had the ability to add the target="cart" attribute, this would be a simple task. However, I can't find it if it does. So, my current solution was to try this: have the PDF file link to a temp_add.php file in the same frame, that file then adds to the cart in other frame, then reverts back to the sender by using the $HTTP_REFERER variable. For some reason, the $HTTP_REFERER variable is NULL when it comes from a PDF file but reports just fine from a regular PHP file or HTML file. Any ideas? Ryan