How does the HTTP Upload feature work?

I want to use the HTTP upload feature to upload the PDFs to a server after creation. On my server, is running Apache/PHP. When I set up the PDFCreator profile, I added Send -> HTTP account with the correct url https://[mysitename. com]/upload.php?key=[my secret key].

However, in my PHP script, no file appears to be uploaded. GET and POST are empty except for the querystring key I specified. The $_FILES array is also empty.

How is the file being uploaded, and why can't PHP see it?

Thanks!