You can surround existing PDF content with your own marked content
block. You may want to start from ElementEdit sample (http://
www.pdftron.com/pdfnet/samplecode.html#ElementEdit). In your case you
will pass-through all elements but you would inject some marked
content. For example:
// Surround an image element with some marked content.
...
element_writer.Flush();
element_writer.WriteString("/Figure <</MCID 0 >> BDC");
element_writer.WriteElement(image_element);
element_writer.WriteString("EMC");
...
For a relevent discussion please see
http://groups.google.com/group/pdfnet-sdk/t/995caad66163d272