How to write code for download file when click on Click here to download link ?

<asp:DataList ID="DataList1" runat="server" RepeatColumns="1"
       RepeatDirection="Horizontal" CellPadding="2" CellSpacing="2" GridLines="Horizontal"
           ForeColor="" Font-Bold="True" Width="100%" RepeatLayout="Flow">
       <ItemTemplate>
          <table width="100%" style="font-family:Arial;font-weight:normal;color:#4b4b4b;font-size:12px;border-bottom:1px solid #CDCDCD;"><tr><td style="width:20%"><b>Class:</b></td><td>
          <asp:Label ID="ClassLabel" runat="server" Text='<%# Eval("Class") %>' />
           </td></tr>
           <tr><td>
           <b>Subject:</b></td><td>
           <asp:Label ID="SubjectLabel" runat="server" Text='<%# Eval("Subject") %>' />
          </td></tr>
           <tr><td><b>Topic:</b></td><td>
           <asp:Label ID="TopicLabel" runat="server" Text='<%# Eval("Topic") %>' />
          </td></tr>
          <tr><td>
           <b>Description:</b></td><td>
           <asp:Label ID="DescriptionLabel" runat="server" Text='<%# Eval("Description") %>' />

           <asp:LinkButton ID="lbtnd" runat="server" OnClick="lbtnd_Click">Click Here To Download</asp:LinkButton>
           </td></tr>
         </table>
       </ItemTemplate>

           <SeparatorStyle BorderStyle="None" />

       </asp:DataList>

protected void lbtnd_Click(object sender, EventArgs e)
    {
        LinkButton lbtn = (LinkButton)sender;
        topic2 = lbtn.Text;
      
        SqlCommand cmd = new SqlCommand("select Fileurl from OE_FileUpload where Description ='" + topic2 + "'", con);
        con.Open();
        fileurl = cmd.ExecuteScalar().ToString();
        string Vpath = "~/UploadData/" + fileurl;
        string ppath = Server.MapPath(Vpath);
        string name = Path.GetFileName(ppath);
        string ext = Path.GetExtension(ppath);
        string type = "";
        if (System.IO.File.Exists(ppath))
        {
            if (ext == ".pdf" || ext == ".PDF")
            {
                type = "Application/pdf";
            }
            else if (ext == ".JPG" || ext == ".jpg")
            {
                type = "Application/jpg";
            }
            else if (ext == ".doc" || ext == ".DOC")
            {
                type = "Application/doc";
            }
            else if (ext == ".docx" || ext == ".DOCX")
            {
                type = "Application/docx";
            }
            else if (ext == ".xls" || ext == ".XLS")
            {
                type = "Application/xls";
            }
            else if (ext == ".xlsx" || ext == ".XLSX")
            {
                type = "Application/xlsx";
            }
            else if (ext == ".zip" || ext == ".ZIP")
            {
                type = "application/x-zip-compressed";
            }
            //else if (ext == ".rar" || ext == ".RAR")
            //{
            //    type = "application/x-rar-compressed";
            //}
            else if (ext == ".rar" || ext == ".RAR")
            {
                type = "application/octet-stream";
            }
            else if (ext == ".txt" || ext == ".TXT")
            {
                type = "application/text";
            }

            if (type != "")
            {
                Response.Clear();
                Response.ClearHeaders();
                Response.ContentType = type;
                Response.AppendHeader("content-disposition", "attachment; filename=" + name);
                FileInfo info = new FileInfo(name);
                Response.Flush();
                Response.WriteFile(ppath);
                // Response.End();
                HttpContext.Current.ApplicationInstance.CompleteRequest();
            }
        }
        else
        {

        }

    }