How to add watermark to a PDF file from a SharePoint 2010 Workflow?
Excellent article and instructions from Murali
The solution presented below executes a workflow whenever a PDF file is added or updated. As part of the workflow it loads portrait and landscape based watermarks and applies the relevant template to each page of the document. The workflow author can specify if the watermark goes in the foreground or the background, the SharePoint location of the watermarks as well as the level of transparency. As the code is well documented it is easy to make further changes and customizations, e.g. the PDF file that needs to be watermarked may not be the file the workflow is acting on.