In the KDE 4.x version of okular, one could zoom by pressing mid mouse button while in browse mode, and then moving the cursor. That functionality is no longer there in the KF5-based version. I think this feature is very useful, because it is easily accessible and doesn't have zoom levels.
Works fine here, are you sure you're on the correct mouse mode?
This bug has had its resolution changed, but accidentally has been left in NEEDSINFO status. I am thus closing this bug and setting the status as RESOLVED to reflect the resolution change.