Today I found online saying that Okular is a very good document viewer. So I installed it from the software center. After the installation, Okular could not open any PDF files. Actually it could not open file. It always complained that "Cannot find a plug in..." and "Could not open ... pdf file".
I looked up on this site, and found somebody said we should delete the related configuration files under
~/.kde/share/. But I didn't found those files there.
BTW, I'm using the Unity desktop.