At the time of writing the RadCartesianChart for UWP does not support export to pdf feature out of the box. This is why it is up to the user to decide how this should be implemented. One possible approach would be to take a picture of the charting component and export the picture instead of the control itself.
After a swift internet search I was able to find this thread
which may be helpful in your scenario.
Pavel R. Pavlov
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items