Try placing the file in a non-DNN folder off the root of your site. Then reference the file via an http link. I am curious to see the difference.
Now I know this does not matter much, but I have adobe reader installed and when I open a pdf link in either FireFox or IE, it always asks me to open or save it. It never opens in the browser for me and this is for any site I go to, not just DNN sites.
For reference, can you copy the link of one that does not work as you would expect and one that does work as you expect? I want to see if there is much of a difference there