To watermark a document in a LaTeX document using pdftex add the following lines to your LaTeX document preamble:

\usepackage{draftwatermark}
\SetWatermarkLightness{ 0.9 }
\SetWatermarkText{\today}
\SetWatermarkScale{ 3 }

First we invoque the LaTeX package draftwatermark. Then, as a watermark is something that appears transparent when viewed, the code defines the lightness of the watermark. Next, the text of the watermark is defined — in the example above I’m using the date. You can choose CONFIDENTIAL or SECRET or any watermark text you might need for your projects. And finally, the scale of the watermark on the document is set. Play with this value to test the coverage of the watermark in the document.

This is a basic example for watermarking a document. If you need more options the draftwatermark package has many and you should read the documentation for all watermarking options and features.