You can contact the Okular team in many ways:
Mailing list: To coordinate okular development, we use the okular-devel mailing list at kde.org. You can use it to talk about the development of the core application, and feedback about existent or new backends is appreciated.
IRC: For general chat, we use the IRC #okular and #kde-devel on the Freenode network. Some of the Okular developers can be found hanging there.
Matrix: The aforementioned chat can also be accessed over the Matrix network via #okular:kde.org.
Forum: If you prefer to use a forum you can use the Okular forum inside the bigger KDE Community Forums.
Bugs and Wishes: Bugs and Wishes should be reported to the KDE bug tracker. If you want to help, you can find a list of top bugs here.