The real issue is finding a docx to PDF converter which supports it.
The XSL FO based converter built into docx4j doesn't, though you may be able to add support which converts a watermark image to fo:region-body/@background-image easily enough; see further pdf-output-f27/watermark-w-pict-v-shape-v-imagedata-t2168.html
A text based watermark would need to be handled differently.
Thanks for the quick reply!! we are generating the Docx and pdf file from an html page. we are converting this html file to FO, so that we don't have any direct control to the FO . So it would be great if you could give me corresponding change that should be made to the html page to get the desired output. Thank you