How to replace specific word in pdf using itextsharp C#.net

//Call below function to create a white image over the text
//you want to delete
RemoveText();

//Call below function to create a new text over the deleted one
string strSource = "D:\\test.pdf";
CreatePDFTemplateMSnew(strSource);


public void RemoveText()
{
   //Path to where you want the file to output
   string outputFilePath = "D:\\test.pdf";
   //Path to where the pdf you want to modify is
   string inputFilePath = "D:\\input.pdf";
   try
   {
      using (Stream inputPdfStream = new FileStream(inputFilePath, FileMode.Open, FileAccess.Read, FileShare.Read))
      using (Stream outputPdfStream = new FileStream(outputFilePath, FileMode.Create, FileAccess.Write, FileShare.ReadWrite))
      using (Stream outputPdfStream2 = new FileStream(outputFilePath, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite))
      {
         //Opens the unmodified PDF for reading
         PdfReader reader = new PdfReader(inputPdfStream);
         //Creates a stamper to put an image on the original pdf
         PdfStamper stamper = new PdfStamper(reader, outputPdfStream); //{ FormFlattening = true, FreeTextFlattening = true };
        		
         //Creates an image that is the size i need to hide the text i'm interested in removing
         iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance(new Bitmap(130, 20), BaseColor.WHITE);
         //Sets the position that the image needs to be placed (ie the location of the text to be removed)
         //txtX.Text = 33,txtY.Text = 708
         image.SetAbsolutePosition(Convert.ToInt16(txtX.Text), Convert.ToInt16(txtY.Text));
         //Adds the image to the output pdf
         stamper.GetOverContent(1).AddImage(image, true);
         //Creates the first copy of the outputted pdf
         stamper.Close();
      }
   }
   catch (Exception ex)
   {
   }
}

//Create a PDF from existing and with a template
private void CreatePDFTemplateMSnew(string strSource)
{
   string oldFile = strSource;

   // open the reader
   PdfReader reader = new PdfReader(oldFile);
   iTextSharp.text.Rectangle size = reader.GetPageSizeWithRotation(1);
   Document document = new Document(size);

   // open the writer
   MemoryStream ms = new MemoryStream();
   PdfWriter writer = PdfWriter.GetInstance(document, ms);
   document.Open();

   // the pdf content
   PdfContentByte cb = writer.DirectContent;
   // select the font properties
   BaseFont bf = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
   cb.SetColorFill(BaseColor.DARK_GRAY);
   cb.SetFontAndSize(bf, 12);
   // create the new page and add it to the pdf
   PdfImportedPage page = writer.GetImportedPage(reader, 1);
   cb.AddTemplate(page, 0, 0);
   // write the text in the pdf content
   cb.BeginText();

   /*
   Paragraph paragraph = new Paragraph();
   paragraph.Alignment = Element.ALIGN_JUSTIFIED;
   Phrase pharse = new Phrase();
   Chunk chunk = new Chunk(txtPdf.Text);
   pharse.Add(chunk);
   paragraph.Add(pharse);
   document.Add(paragraph);
   */
   string text = txtText.Text; //"Put Text"
   int intAlign = Convert.ToInt16(txtAlign.Text); //0
   int intX = Convert.ToInt16(txtX.Text); //35
   int intY = Convert.ToInt16(txtY.Text); //714
   int intRotation = Convert.ToInt16(txtRotation.Text); //0
   // put the alignment and coordinates here
   cb.ShowTextAligned(intAlign, text, intX, intY, intRotation);
   cb.EndText();
   //cb.BeginText();
   //text = "Other random blabla...";
   // put the alignment and coordinates here
   //cb.ShowTextAligned(2, text, 100, 200, 0);
   //cb.EndText();

   //document.newPage(); 
   //PdfImportedPage page2 = writer.GetImportedPage(reader, 2); 
   //cb.AddTemplate(page2, 0, 0);
   writer.CloseStream = false;
   if (document.IsOpen()) document.Close();
   ms.Position = 0;
   DownloadAsPDF(ms);

   // close the streams and voilá the file should be changed :)
   if (writer != null) writer.Close();
   if (ms != null) ms.Close();
   if (reader != null) reader.Close();
}

private void DownloadAsPDF(MemoryStream ms)
{
   string attachment = "attachment; filename=Report_" + DateTime.Now.ToString("ddMMyyyyhhmmss") + ".pdf";
   Response.Clear();
   Response.ClearContent();
   Response.ClearHeaders();

   Response.AppendHeader("Content-Disposition", attachment);
   Response.ContentType = "application/pdf";
   Response.OutputStream.Write(ms.GetBuffer(), 0, ms.GetBuffer().Length);
   Response.OutputStream.Flush();
   Response.OutputStream.Close();
   Response.End();
   ms.Close();
}