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;
// Draws 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();