title: How to stamp and sign a pdf ALREADY signed without invalidating previous signature
body: <p>Some plugins and programs can do this; is there any example or tutorial how to do this using .net and itextSharp??</p>
  
  <p>I need to take a signed pdf and stamp an imagen on all pages and also add another signature.
  The first signature on the document must be valid when open the file.</p>
  
  <p>Thanks in advance.</p>