Make PDF viewer working again in public view

This little PR makes the PDF viewer working again in the public view. It's neither beautiful code nor some black magic. It just does it job (TM).

I know that we're already behind the feature freeze but this change is minimal and can be easily tested. And I *really* need this feature ;-)
(Our preview picture for PDFs is just ... let's call it "suboptimal" ...) 

@karlitschek 
I'd prefer using `addClass('hidden')` as it makes the code testable with JS Unit (if you ever add it later...)