In order to do this you need 2 files. One file containing the watermark with the username and the other file is the file which you want to put the watermark on.
Do you want to put the username as a watermark on the PDF file? Then first create a pdf file with the username using a document template in mendix and use the generated PDF file to overlay your file.
Thanks, I did . may I know how I can retrieve this file on page? where this newly generated file stay?
I would like to suggest you PDF Watermark Tool by which you can simply put your image, text, etc as watermark on your PDF files on a Windows PC without getting any error. Visit here to download this tool:- https://www.osttopstapp.com/pdf-watermark.html