It's kind of nice that some apps install themselves as default for opening files, but it would be even nicer if there were some easy way for users to have a say.. for example, it's fine with me that Quiver took over the photos, but I would like to leave the original PDF viewer for PDF files for now. Still want to keep evince installed though.

(I should add that obviously on my N800 Evince managed to install the "takeover" setting, for some reason I don't observe the OP's reported problem and thus I can't help with that.)