how large are the PDFs you are trying to upload?
it could be that the size of the PDF is causing the upload to fail. There are a number of PHP server settings that will effect the maximum size of uploaded files, try uploading a smaller pdf, one that is the same size as the jpg you tried. If the upload works with the smaller pdf, then the problem is being caused by the server settings.
see the following page for details on php.ini settings that effect the allowed upload file size: