I have done a bit of digging on this and the Digital File Pro code should force the download of the file. It is very similar to the code examples on php.net for this type of download. In my testing and searching I found some other code that may help in the download process. I have made an example script for this code so that you can do a test.
In this example you will need to fill in a file name and the file system path to that file. Place this code on a new page then after you fill in the values correctly load it to your live server and test it out. Let us know what the result is.
$fileName = ""; //the name of the file, example: yourfile.pdf
$path = "" . $fileName; //the file system path to the file, example: C:\\Inetpub\\wwwroot\\
header('Content-Disposition: attachment; filename="' . $fileName . '"');
//these next four lines are not in the DFP code
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
//down to here
//these next two lines are not in the DFP code also
// to here