I am sure that Okular has a following feature:
When I have already opened some .pdf file in Okular and then I will double-click on the another different .pdf file, Okular will open the second file in card, like a browser opens new page, instead of starting new Okular.
I just can't find how to enable that feature. Thanks for any help and I aplogize for my english if I made any mistakes. Anyway I hope you will know what I meant.