In that case, let your user use Firefox by showing (better browsed with FF). Also, let them use the PDF Download extension (free from Mozilla). Once configured all PDF is automatically downloaded.

Since not all users will know what to do with just what I suggested above. And since you really wanted to do just what you say you wanted. Download PDF Download and modify portions to make it work the way you wanted and have users download that modified extension.

A hell lot of job but that's what web-developer means. (Ever seen a Chef relying on Waiter to cook the meal)