How to convert color image to black and white/gray image in WinForms PDF?

public Bitmap ConvertToBlackandWhite(Bitmap image)
{
 Bitmap bitmap = new Bitmap(image);
 for (int i = 0; i < bitmap.Width; i++)
 {
   for (int j = 0; j < bitmap.Height; j++)
   {
     int ser = (bitmap.GetPixel(i, j).R + bitmap.GetPixel(i, j).G + bitmap.GetPixel(i, j).B) / 3;
     bitmap.SetPixel(i, j, Color.FromArgb(ser, ser, ser));
   }
 }
return bitmap;
}

private void button1_Click(object sender, EventArgs e)
{ 
 //Loading the existing document.
 PdfLoadedDocument ldoc = new PdfLoadedDocument("input.pdf");
 
 PdfLoadedPageCollection loadedPages = ldoc.Pages;
 
 //iterate through the page.
 foreach (PdfLoadedPage lpage in loadedPages)
 {
  //Extracting images form Page.
  Image[] images = lpage.ExtractImages();
  for (int i = 0; i < lpage.ImagesInfo.Length; i++)
   {
    Bitmap map = new Bitmap(images[i]);
   //Converting color image to black white/gray image.
   PdfImage image = PdfImage.FromImage(ConvertToBlackandWhite(map));
   //Replace the image in the page.
   lpage.ReplaceImage(i, image);
  }
 }
 
 //saving the document
 ldoc.Save("output.pdf");
 ldoc.Close(true);
}