title: Detect Changes on Signed PDF that was done between signatures
body: <p>I'm developing an application that should verify signatures of pdf files. The application should detect full history of updates done on the file content before each signature is applied.
  For example:</p>
  <ol>
  <li>Signer 1 signed the plain pdf file</li>
  <li>Signer 2 added comment to the signed file, then signed it</li>
  </ol>
  <p>How can application detect that Signer 2 added a comment before his signature.</p>
  <p>I have tried to use itext and pdfbox</p>