Found this nifty action hidden in Mac OS Automator to watermark PDF documents. Start your workflow with Ask for Finder Items, then choose Watermark PDF Documents. You then select the image file to use as the watermark and control the layer, offset, scale, angle and opacity settings. The rest is up to you, noting you can add actions to encrypt the PDF or edit the metadata among other things. Great way to avoid the full version of Acrobat or other 3rd party solutions.