You can add text or image watermarks to the document via TextWatermark and ImageWatermark classes. You can customize the watermarks with wide range of options e.g. position, size, rotation, opacity, color. Downloading as PDF will also include the same watermarks.

Watermarks can be applied to specific pages via PageRange property. The range is specified as a string in the following forms or combination thereof:

5 | single value
1,2,3,4,5 | sequence of values
1-5 | closed range
-5 | open range (converted to a sequence from 1 to 5)
1- | open range (converted to a sequence from 1 to final page)
all | keyword representing all page numbers.
odd | keyword representing odd page numbers.
even | keyword representing even page numbers.