Hi Sahil,
For Web Services to work the PDF must have Reader Rights applied to it to enable additional functionality in Reader. This is why you install a Reader Rights credential when you want to implement interactive scenarios. In Designer Preview the Reader Rights do not get applied to a PDF so you can't test web services in Designer with Reader.
What people usually do is write a simple ABAP program that renders the PDF and allows to download it for testing.
Juergen