How to replace the existing text in the PDF document?

Dictionary<int, List<RectangleF>>  matchedTextbounds = new Dictionary<int, List<RectangleF>>();

//Loads the PDF document.

PdfLoadedDocument loadedDocument = new PdfLoadedDocument("../../Data/PDFDocument.pdf");

// Retrieves the bounds of the searched text using FindText method.

loadedDocument.FindText("Pdf", out  matchedTextbounds);

PdfLoadedPage page = null;

// Font which is used to draw the replacement text.

PdfFont font = new PdfTrueTypeFont(new Font("Arial", 11));

// Iterate throught the pages of the PDF document.       

for (int i =0;i< matchedTextbounds.Count;i++)

{

    page= (PdfLoadedPage)loadedDocument.Pages[i];

    // Iterate through the instance of the searched text in page.

    foreach (RectangleF rectangle in matchedTextbounds[i])

      {

         // Draws the white filled rectangle over the searched text.

         page.Graphics.DrawRectangle(PdfBrushes.White, rectangle);

         // Draws the replacedment text with respect to the font and bounds of the

            searched text.

         page.Graphics.DrawString("PDF", font, PdfBrushes.Black, rectangle.X-2,

         rectangle.Y-2);

       }

 }

 // Saves the PDF document to specified location.

 loadedDocument.Save("../../Data/PDFDocument_Replaced.pdf");