You can use the Watermark PDF documents action in Automator to add an image on every page of a PDF file. You can specify location, scale, rotation, and opacity.
Manually fixing the workflow for Mac OS X Lion
Set up the entire automator workflow as you would, but skip adding the watermark image (you can't). Save the workflow to a file. Right-click the workflow in Finder and select Show Package Contents and navigate into
document.wflow in a text editor, e.g. TextMate.
The actual parameters and their values are stored in the
ActionParameters dictionary. The relevant key is
filenames. By default, it looks like this:
Just edit the
<array/> declaration and add all files as
<string>s, like so:
Save the file, and reopen the workflow in Automator. The image will be correctly listed, and you can fix its position and other properties, before you apply it.
Remember that the workflow does not replace the input file as many others do, the output is written to a temporary file. Use the Move Finder Items action afterwards.
Your printer might support a watermark feature via the print dialog. Check for a watermark option in the popup list of the detailed print dialog.