The default Document Viewer in Ubuntu 14.04 is Evince. Recently it started to have problems with many PDF files that are opened by other PDF viewers (Okular, Acroread) without problems. Actually Evince opens them, but the window remains invisible. I tried to change the PDF viewer, but Ubuntu has apparently removed all means to do so in version 14.04 short of going into the command line and editing configuration files by hand: what a progress! In previous versions of the File Manager, it was possible to right-click a file and after selecting "Open with" > "Other Application" to choose "Set as default". Now the only way to do so is to edit ~/.local/share/applications/mimeapps.list, replacing
[Default Applications] with
[Default Applications] I guess it is time to switch back to OpenSuSE...