Are there any files in the PDF/ upload directory?
Can those be accessed through the browser?
I tried uploading an image named 1.jpg using your form, then browsing to :
this gave an error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
In DFP, you should leave the file setting at the default:
to let dfp manage the name of the file that is uploaded.
Also, add the following at line 1 to make sure a PHP error is not occuring that is preventing the file upload: