Q: Is there a way to remove a watermark from an existing PDF document
i.e. one not created using PDFNet?
A: If the watermark/stamp is represented as a annotation you could use
page.AnnotRemove() to remove the annotation object from the page.
In watermark/stamp is part of PDF content stream you could remove
graphical elements as shown in ElementEdit sample project (http://
www.pdftron.com/pdfnet/samplecode.html#ElementEdit). The only tricky
part is how to identify the part of the content stream that represents
the stamp. If you only need to remove your own stamps the task in not
very hard (for relevant discussion you may want to search for the
article with the tile - “Help with creating / removing a header/footer
in PDF” in this forum). If the stamp is applied using a third party
tool you could also remove the content provided there is a way for you
to distinguish stamp content from the rest of the page.