Did you also install the squillions of dependencies that okular has? Presumably so unless you force-installed it.
The other thing is, I ran into this issue installing a deb from the debian repos, and the only way I managed to get around it was to get a slightly newer version (see this search for example:
http://packages.debian.org/search?su...eywords=okular)
Except the newer builds are only available for armel so then you need to use --force-architecture to install it:
sudo dpkg -i --force-architecture [deb filename]
Note that this may be dangerous so don't blame me if you need to reflash your Q7 or something, but in general it's fine, arm and armel are basically the same.
You might find that the newer version needs newer versions of everything else too, like libc and stuff, if that's the case I'd just give up, it's not worth trying to replace all of those since they are used by other programs.