How to replace pages in a PDF document?

//Existing pdf with file path.
 string file1 = "../../Data/imageDoc.pdf";
 
//Loading File1
 PdfLoadedDocument ldDoc1 = new PdfLoadedDocument(file1);
 
 //Gets the page collection of loaded document
 PdfLoadedPageCollection pageCollection = ldDoc1.Pages;
//Removes the page at index zero
 pageCollection.RemoveAt(0);
 //Initializes font.
 PdfFont font = new PdfStandardFont(PdfFontFamily.Helvetica, 12);
 //Creates a new Pdf document
 PdfDocument doc = new PdfDocument();
 //Adds a new page
 PdfPage page = doc.Pages.Add();
 PdfGraphics g = page.Graphics;
 //Draws string in the new page.
 g.DrawString("Hello", font, PdfBrushes.Black, new PointF(0, 0));
 //Creates template of replacing page.
 PdfTemplate template = page.CreateTemplate();
 
 //Inserts new page at index zero in loaded document
 PdfPage newLoadedPage = pageCollection.Insert(0) as PdfPage;
  PdfGraphics lgraphics = newLoadedPage.Graphics;
  //Draw the template of the replacing page in the newly inserted loaded document page graphics.
 lgraphics.DrawPdfTemplate(template, PointF.Empty, page.GetClientSize());
 
 //Save the document and dispose it
 ldDoc1.Save("sample.pdf");
 ldDoc1.Close();