Adobe reader PDF viewer overlay issue (Windows)

I have an issue regarding to opening PDF on windows with Adobe Reader (might be on other platform but I didn't test yet).

This is reproductible on Qt Browser demo. I used Adobe Reader XI (11.0.09, last version).

When opening a pdf in the tab browser, it works fine but when switching the overlay created by the pdf reader stay on top of the page (pdf_viewer_overlay.png). If the app was not taking the full screen space and I maximised it, I can see the page below (pdf_viewer_resize.png).

It's quite annoying as you can imagine, is there any way to define pdf as unsuported (force download) in webkit so they can then decided to open them separatly (in another process) and not directly in the browser?