Okular flatpak missing unrar (to read comics books for example)

Well, I think that adding unrar to the installation of okular would be a great idea since it needs it for somethings.

Could you open an issue at https://github.com/flathub/org.kde.okular?

Actually, did it for you: https://github.com/flathub/org.kde.okular/issues/61

Anything technical (which is not an “user” issue or discussion) better suits issues of respective app.