title: Change Title metadata of pdf so that it equals the filename
body: <p>I'm searching for any <code>.pdf</code> in a directory that doesn't currently have a <code>Title</code> property and replacing it with the file name of the document.  I've seen a couple of instances in which this is done such as <a href="https://stackoverflow.com/questions/7380837/set-metadata-in-itextsharp">here</a>, but it looks like they're replacing the entire document.  I just want to add a <code>Title</code> to the document without completely rewriting it.  </p>
  
  <p>How can I do this with iTextSharp?</p>