Okular doesn't permit users to print on both sides (duplex printing) a crucial feature
same goes for every other kde app that edits text or views documents
I do duplex printing with Okular every day, so I'd say that your problem is somewhere in the lower layers. CUPS misconfiguration, maybe?
Created attachment 111948 [details] no duplex option 1
Created attachment 111949 [details] no duplex option 2
Well after extensive searching in print menu dialogs I could NOT locate a duplex option. Refer to screen shots.
Had to use google chrome for duplex.
Your first screenshot shows a "Duplex Printing" section. By saying there is no duplex option, do you mean that everything except for "None" is greyed out? (it's not visible on the screenshot) Also, can you please run the following command in a terminal and paste the output here (it lists the CUPS options available for your printer): lpoptions -p Officejet_Pro_6830
Created attachment 111951 [details] command
It is supported.