How to add a watermark to a PDF file?

Question

I\'m using C# and iTextSharp to add a watermark to my PDF files:

Document document = new Document();
PdfReader pdfReader = new PdfReader(strFileLocation);
Pd         


        

Answer 1

Although I don't know the specifics of iTextSharp, likely on the pages where your image is not showing, the previous PDF content has modified the current transformation matrix such that whatever you put on the page is moved off the page.

This can be fixed by emitting a gsave operator before the original page content and emitting a grestore operator after the original page content (but before yours). This, however may not fix all cases with a PDF document that modifies the CTM does a gsave and no grestore. This is not supposed to happen in theory, according to the PDF specification:

Occurrences of the q and Q operators shall be balanced within a given content stream (or within the sequence of streams specified in a page dictionary’s Contents array).

but I can tell you from experience that this is not the case in practice.