cannot implicitly convert type arraylist to generic.list itextsharp.text.element

private void generate_pdf(string path, string file, bool download, string text)
   {
       var document = new Document();
       try
       {
           if (download)
               PdfWriter.GetInstance(document, Response.OutputStream);
           else
               PdfWriter.GetInstance(document, new FileStream(path + file, FileMode.Create));

           StringBuilder strB = new StringBuilder();
           document.Open();
           repeat();


           if (text.Length.Equals(0))
           {
               using (StringWriter sw = new StringWriter(strB))
               {
                   using (HtmlTextWriter hw = new HtmlTextWriter(sw))
                   {
                       dt = (DataTable)Session["dataItem"];

                       if (dt.Rows.Count > 0)
                       {
                           DataRow drall;

                           PlaceHolder pholder = new PlaceHolder();
                           pholder.Controls.Add(new LiteralControl("<div ><table><tr><td colspan='6'  align='center' style='font-weight:bold;font-size:18px'>REPORT on " + Text_From.Text + " to " + Text_To.Text + "</td></tr></table></div>"));
                           pholder.Controls.Add(new LiteralControl("<table border='1'><tr style='background:#dadada;font-weight:bold;'><td align='center'>S.No</td>"));
                           pholder.Controls.Add(new LiteralControl("<td align='center'>Order ID</td>"));
                           pholder.Controls.Add(new LiteralControl("<td align='center'>Name</td>"));
                           pholder.Controls.Add(new LiteralControl("<td align='center'>Password</td>"));
                           pholder.Controls.Add(new LiteralControl("<td align='center'>Imagename</td>"));
                           pholder.Controls.Add(new LiteralControl("<td align='center'>Updated by</td>"));
                           pholder.Controls.Add(new LiteralControl("<td align='center'>Image</td>"));
                           pholder.Controls.Add(new LiteralControl("</tr>"));

                           for (int i = 0; i < dt.Rows.Count; i++)
                           {
                               int sno = i + 1;
                               drall = dt.Rows[i];
                               pholder.Controls.Add(new LiteralControl("<tr>"));
                               pholder.Controls.Add(new LiteralControl("<td >" + sno + "</td>"));
                               pholder.Controls.Add(new LiteralControl("<td>" + drall["id"].ToString() + "</td>"));
                               pholder.Controls.Add(new LiteralControl("<td align='right'>" + drall["username"].ToString() + "</td>"));
                               pholder.Controls.Add(new LiteralControl("<td align='right'>" + drall["password"].ToString() + "</td>"));
                               pholder.Controls.Add(new LiteralControl("<td align='right'>" + drall["imagename"].ToString() + "</td>"));
                               pholder.Controls.Add(new LiteralControl("<td align='right'>" + drall["updatedby"].ToString() + "</td>"));
                               pholder.Controls.Add(new LiteralControl("<td align='right'>" + drall["uploadimage"].ToString() + "</td>"));
                               pholder.Controls.Add(new LiteralControl("</tr>"));
                           }

                           pholder.Controls.Add(new LiteralControl("</table>"));
                           pholder.RenderControl(hw);
                       }
                   }
               }
           }

           else

               strB.Append(text);

           using (TextReader sReader = new StringReader(strB.ToString()))
           {

               List<IElement> list = HTMLWorker.ParseToList(sReader, new StyleSheet());   // here i got error
               foreach (IElement elm in list)
               {
                   document.Add(elm);
               }
           }


   }
       catch (Exception ex)
       {

       }
       finally
       {
           document.Close();
       }

   }

protected void btn_pdf_Click(object sender, EventArgs e)
   {

       try
       {

           string file = "reports" + DateTime.Now.Ticks + ".pdf";
           Response.Clear();
           generate_pdf("", file, true, "");
           Response.ContentType = "application/pdf";
           Response.AddHeader("content-disposition", "attachment; filename=" + file);
           Response.Flush();
           Response.End();

       }

       catch(Exception ex)
       {
           throw ex;
       }

   }