title: Signature in PDF becomes invalid after modification in pdf
body: <p>I have Digitally Signed a PDF named as "Original" with Succes Message : "Signed and all signatures are valid."
  When i try to modify that pdf's data using below code and open the document named as "NewlyCreated" ,signatures become invalid
  with Error Message : AtLeast one signature is valid</p>
  
  <pre><code>public class Program
  {
      public const String src = @"C:\Original.pdf";
      public const String dest = @"C:\NewlyCreated.pdf";
      public void createPdf1(String dest)
      {
          PdfReader reader = new PdfReader(src);
          PdfDocument pdfDoc = new PdfDocument(reader,new PdfWriter(dest),new StampingProperties().UseAppendMode());
          PageSize ps = pdfDoc.GetDefaultPageSize();
          Paragraph p = new Paragraph("This is the text added in the rectangle.");
          PdfCanvas canvas = new PdfCanvas(pdfDoc.GetFirstPage());
          Rectangle rect = new Rectangle(ps.GetWidth() - 90, ps.GetHeight() - 100, 50, 50);
          new Canvas(canvas, pdfDoc, rect)
              .Add(p);
          canvas.Rectangle(rect);
          canvas.Stroke();
          pdfDoc.Close();
      }
  
      public static void Main(string[] args)
      {
          Program objProgram = new Program();
          objProgram.createPdf1(dest);
      }
  }
  </code></pre>
  
  <p>How can i modify pdf without invalidating its signature using iText7</p>