How to merge multi pdf files in one pdf ?

protected void btnPrintMultiplePdf_click(object sender, EventArgs e)
    {
        if (ValidateForMultiplePDF() == true)
        {
            #region Declare Temp Variables..!!
            CheckBox chkList = new CheckBox();
            HiddenField HidNo = new HiddenField();

            string Multi_fofile, Multi_listfile;
            Multi_fofile = Multi_listfile = "";
            Multi_fofile = Server.MapPath("PDFRNew");
            #endregion

            for (int i = 0; i < grdRnew.Rows.Count; i++)
            {
                #region Find Grd Controls..!!
                CheckBox Chk_One = (CheckBox)grdRnew.Rows[i].FindControl("chkOne");
                Label lbl_Year = (Label)grdRnew.Rows[i].FindControl("lblYear");
                Label lbl_No = (Label)grdRnew.Rows[i].FindControl("lblCode");
                #endregion

                if (Chk_One.Checked == true)
                {
                    HidNo .Value = llbl_No .Text.Trim()+ lbl_Year .Text;

                    if (File.Exists(Multi_fofile + "\\" + HidNo.Value.ToString() + ".pdf"))
                    {
                        #region Get Multiple Files Name And Paths..!!
                        if (Multi_listfile != "")
                        {
                            Multi_listfile = Multi_listfile + ",";
                        }
                        Multi_listfile = Multi_listfile + Multi_fofile + "\\" + HidNo.Value.ToString() + ".pdf";

                        #endregion
                    }
                }
            }

            #region For Generate Multiple Pdf..!!
            if (Multi_listfile != "")
            {
                String[] Multifiles = Multi_listfile.Split(',');
                string DestinationFile = Server.MapPath("PDFRNew") + "\\Multiple.Pdf";
                MergeFiles(DestinationFile, Multifiles);

                Response.ContentType = "pdf";
                Response.AddHeader("Content-Disposition", "attachment;filename=\"" + DestinationFile + "\"");
                Response.TransmitFile(DestinationFile);
                Response.End();
            }
            else
            {

            }
            #endregion
        }
    }

    private void MergeFiles(string DestinationFile, string[] SourceFiles)
    {
        try
        {
            int f = 0;
            /**we create a reader for a certain Document**/
            PdfReader reader = new PdfReader(SourceFiles[f]);
            /**we retrieve the total number of pages**/
            int n = reader.NumberOfPages;
            /**Console.WriteLine("There are " + n + " pages in the original file.")**/
            /**Step 1: creation of a document-object**/
            Document document = new Document(reader.GetPageSizeWithRotation(1));
            /**Step 2: we create a writer that listens to the Document**/
            PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(DestinationFile, FileMode.Create));
            /**Step 3: we open the Document**/
            document.Open();
            PdfContentByte cb = writer.DirectContent;
            PdfImportedPage page;
            int rotation;
            /**Step 4: We Add Content**/
            while (f < SourceFiles.Length)
            {
                int i = 0;
                while (i < n)
                {
                    i++;
                    document.SetPageSize(reader.GetPageSizeWithRotation(i));
                    document.NewPage();
                    page = writer.GetImportedPage(reader, i);
                    rotation = reader.GetPageRotation(i);
                    if (rotation == 90 || rotation == 270)
                    {
                        cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(i).Height);
                    }
                    else
                    {
                        cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
                    }
                    /**Console.WriteLine("Processed page " + i)**/
                }
                f++;
                if (f < SourceFiles.Length)
                {
                    reader = new PdfReader(SourceFiles[f]);
                    /**we retrieve the total number of pages**/
                    n = reader.NumberOfPages;
                    /**Console.WriteLine("There are"+n+"pages in the original file.")**/
                }
            }
            /**Step 5: we Close the Document**/
            document.Close();
        }
        catch (Exception e)
        {
            string strOb = e.Message;
        }
    }

    private bool ValidateForMultiplePDF()
    {
        bool chkList = false;

        foreach (GridViewRow gvr in grdRnew.Rows)
        {
            CheckBox Chk_One = (CheckBox)gvr.FindControl("ChkSelectOne");
            if (Chk_One.Checked == true)
            {
                chkList = true;
            }
        }
        if (chkList == false)
        {
            divStatusMsg.Style.Add("display", "");
            divStatusMsg.Attributes.Add("class", "alert alert-danger alert-dismissable");
            divStatusMsg.InnerText = "ERROR !!...Please Check At Least On CheckBox.";
            grdRnew.Focus();
            set_timeout();
            return false;
        }
        return true;
    }