PDF Watermark is a free program that can, as the name suggests, add a watermark to 1 or more pages of any PDF file. Choose from text of your choice, or an image, and add it to your file in order to help prevent unauthorised use or distribution.

This is handy if you use any other programs that can generate PDF files, but which don't have the ability to add watermarks.

The download is at http://pdfconverters.net/pdf-watermark-free.html and is an 8 MB file. Despite the download button saying "free trial", the program is actually totally free. And it's malware-free according to VirusTotal. The download site is fully trusted by Web of Trust too.