If you would like to install a good pdf viewer in gnome you might pick a kde application, sure that’s great, but it might not look so good at first.
I like to use okular in gnome but if you just install okular like
> zypper in okular
it will get installed but at the same time all of the icons will be missing from the menus and even the application image wont be there. Sure it is fully functional but it doesn’t look good.
A way to get these icons is to install a kde icon package such as oxygen (if I start okular from the command line it complains a lot about oxygen icons) but a more concrete way to do this is to install the kde base pattern. In my case on tumbleweed I had to install the “kde” pattern (more info could be on  but info could also be outdated, I know this will install a lot of unnecessary things but IMHO it’s safer and easier to do it this way.
> zypper install -t pattern kde
If your are not in openSUSE you should check for a pattern (if your distribution has patterns) like “KDE Desktop environment” or something that could contain the terms “KDE”, “base”, “environment” or on your distribution’s forum for help installing the KDE environment.
If you re in openSUSE and you don’t find this pattern go on yast and search for something similar under patterns.
The result would be something like this.