License code is only for the dmg version downloaded from our website. If you can't find "Enter License", please make sure that you use PDF Reader Pro dmg version.
If there are different versions of PDF Reader Pro on your device, we recommend that you uninstall all, restart your device, and redownload the latest verison from here.
Then enter your license by tapping PDF Reader Pro on the main menu and click Enter License.
Note:
FYI: Difference between PDF Reader Pro.dmg and PDF Reader Pro (Apple Store)