I had searched for this quite some time ago, but got into it again. I really want okular to be able to detach tabs. In dolphin this is possible by right-clicking on the tab, which is really intuitive although pulling it with the mouse out of the current window would be nice as well 
However the last time I searched for this I concluded, that this is more a qt-feature rather than an okular one, from a reddit post which I sadly am not able to find anymore. I have not yet installed plasma 6, so maybe this may be resolved in a few days, but if not: This is a very important feature to dolphin if you handle more than one document at a time on multiple monitors!