I have installed Okular in my Ubuntu for several months and appreciated its annotation functions which allow me more freedom with my own documents. When I want to invoke Okular from the command line, however, it cannot be found by the system. After some investigations, the symbolic link seems to be the handiest solution.
Go to /usr/bin and use ln to make a symbolic link:
$ sudo ln -s /usr/lib/kde4/bin/okular okular
This is my first time to use ln, and it works as expectation. :-)