Watermarks can be added in various ways. If you already have an existing watermark in form of a PDF, you could just use the overlay functionality (see Merging PDFs). Alternatively, a watermark can be added via CSS. The following sample CSS uses a page margin box to add the semi-transparent text "CONFIDENTIAL" above each page:
@page {
@left-middle {
content: "Confidential";
z-index: 100;
font-family: sans-serif;
font-size: 80pt;
font-weight: bold;
color: gray(0, 0.3);
text-align: center;
text-transform: uppercase;
transform: rotate(-54.7deg);
position: absolute;
left: 0;
top: 0;
width: 100%;
height: 100%;
/*
The following property is optional and makes the
watermark text non-selectable.
*/
-ro-pdf-text-rendering: visual-only;
}
}