Hi Kaveri,
With using OverlayPDF you can able to achieve this one, One PDF actual one, Another one you need to generate along with your expected content then using OverlayPDF Javaaction you can able to add watermark.
Also refer the below video I created for you clarification.
How to add watermark in Mendix using OverlayPDFDocument JavaAction
Hope it Helps
Thanks & Regards,
Manikandan K