Hi Naveen,
There is no way to apply a true watermark to a document in OnBase, even when using an OnBase API. The closest thing you can do is to setup an image overlay to your document (which needs to be configured via OnBase Config and not via code), and apply the watermark image over the document that way.
Also, you mentioned that you wish to apply a watermark to a PDF. Overlay functionality in OnBase cannot be done against PDF documents, so if you wish to use the overlay functionality for this, you will need to convert your PDF document to an image file, and then apply the overlay to the image version of the PDF.
Ultimately, I would recommend that you use a third party software or tool to apply watermarks to your PDF files, as they are relatively difficult to do in OnBase.
