Running Kubuntu 14.04 Trusty Tahr LTS, I am trying to have Okular (version 0.19.3) scrolling when pressing the middle mouse button and moving it in any direction to scroll in that direction.
Currently, when the center button is pressed (clicked), the option (when holding and moving the cursor) that I have is zoom in and out.
Okular's maintainer has previously implied there are no plans to change the current behavior as stated on the bugs.kde.org page linked below. Some people that are not satisfied with that have provided patches at the bottom of the bugs.kde.org page linked below that will change Okular's default behavior of zooming while holding the middle mouse button to scrolling: https://bugs.kde.org/show_bug.cgi?id=219121
Perhaps the patch from November of 2013 (from the link above) will help your situation. You can apply the patch to the source of Okular then compile a version that works the way you want. If the patch is for a version of Okular that is too different then you could study the patch and find which code needs to be changed manually. You may find Kompare to be useful for applying patches and comparing source files. Just Google Kompare for more info.
There is good information on compiling Okular instead of installing from binary packages on this page: https://okular.kde.org/download.php