The form fields are not supported in RadPdfProcessing at this point. We have this feature request logged in our backlog and you could vote and subscribe to the related public item
in our feedback portal in order to track its overall progress.
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? Explore the Telerik Feedback Portal
and vote to affect the priority of the items