PdfSignature signature = new PdfSignature(doc, page, cert, "123456");
signature.ConfigureCustomGraphics(DrawGraphics);
 private void DrawGraphics(PdfCanvas g)
{
   PointF point1 = new PointF(10, 20);
   PointF point2 = new PointF(25, 50);
   PointF point3 = new PointF(60, 20);
   PointF[] pntArr = { point1, point2, point3 };
   PdfRGBColor color = new PdfRGBColor(Color.Red);
   g.DrawPolygon(new PdfPen(color), pntArr);
   g.DrawRectangle(new PdfPen(Color.Green), new RectangleF(70, 10, 20, 30));
   g.DrawLine(new PdfPen(Color.Blue), new PointF(120, 50), new PointF(95, 5));
   g.DrawLine(new PdfPen(Color.Blue), new PointF(100, 30), new PointF(130, 20));
 }