This should be possible already since 4.4 with a custom button and process, as processes support conditions:
https://github.com/orbeon/orbeon-forms/wiki/Form-Runner:-Buttons-and-Processes#conditionsHere is how it would work:
- button starts process
- condition checks value in instance
- if condition is satisfied
- run send action
- your service receives the URL of the PDF created
Would this work?
-Erik