Dear Harry,
This can only be achieved by adding some server-side code (in PHP). There are no attributes in HTML to force download.
Here's an example on how to do it, with ready to use code: http://stackoverflow.com/a/364950
Please take care of validation of the filename if you decide to implement it, especially the line with $file = $_GET["file"] .".pdf" needs attention.
Please let me know if you are satisfied with my answer and if I can help you with any other questions you might have.
Regards,
Paweł