add refernce of itext dll file to your project

pdf.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="pdf.aspx.cs" Inherits="pdf" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>Grdview with Arraylist</title>
<style type="text/css">
.Gridview
{
font-family:Verdana;
font-size:10pt;
font-weight:normal;
color:black;
width:300px;
}
    .style1
    {
        width: 100%;
    }
</style>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:GridView ID="gvdata" runat="server" CssClass="Gridview"
        AutoGenerateColumns="False" HeaderStyle-BackColor="#7779AF"
        HeaderStyle-ForeColor="White">
<Columns>
<asp:BoundField DataField="ID" HeaderText="ID"/>
<asp:BoundField DataField ="Name" HeaderText="Name" />
</Columns>

<HeaderStyle BackColor="#7779AF" ForeColor="White"></HeaderStyle>
</asp:GridView>
    <table class="style1">
        <tr>
            <td>
                &nbsp;</td>
            <td>
                <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Search" />
            </td>
        </tr>
        <tr>
            <td>
                &nbsp;</td>
            <td>
                <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td>
                &nbsp;</td>
            <td>
                <asp:ImageButton ID="btnPdf" runat="server" ImageUrl="~/2.jpg"
onclick="btnExcel_Click" />&nbsp;</td>
        </tr>
        <tr>
            <td>
                &nbsp;</td>
            <td>
<asp:GridView ID="gvdata0" runat="server" CssClass="Gridview" AutoGenerateColumns="False"
                    HeaderStyle-BackColor="#7779AF" HeaderStyle-ForeColor="White">
<Columns>
<asp:BoundField DataField="ID" HeaderText="ID"/>
<asp:BoundField DataField ="EmpName" HeaderText="EmpName" />
</Columns>

<HeaderStyle BackColor="#7779AF" ForeColor="White"></HeaderStyle>
</asp:GridView>
            </td>
        </tr>
    </table>
</div>
</form>
</body>
</html>

pdf.aspx.cs

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using iTextSharp.text;
using iTextSharp.text.pdf;
using iTextSharp.text.html.simpleparser;
using System.Data.SqlClient;
using System.IO;
using System.Configuration;

public partial class pdf : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    public override void VerifyRenderingInServerForm(Control control)
    {
        /* Verifies that the control is rendered */
    }
    protected void Button1_Click(object sender, EventArgs e)
    {

        BindGridviewData();
        BindGridviewData1();

    }
    protected void BindGridviewData()
    {
        SqlConnection con = new SqlConnection(Connect.cnst);
        con.Open();
        SqlCommand cmd = new SqlCommand("select [ID],[Name] from exm where ID = '" + TextBox1.Text + "'", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        if (ds.Tables[0].Rows.Count == 0)
        {
            ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
            gvdata.DataSource = ds;
            gvdata.DataBind();
            int columncount = gvdata.Rows[0].Cells.Count;
            gvdata.Rows[0].Cells.Clear();
            gvdata.Rows[0].Cells.Add(new TableCell());
            gvdata.Rows[0].Cells[0].ColumnSpan = columncount;
            gvdata.Rows[0].Cells[0].Text = "No Records Found";
        }
        else
        {
            gvdata.DataSource = ds;
            gvdata.DataBind();
        }
        con.Close();
    }

    protected void BindGridviewData1()
    {
        SqlConnection con = new SqlConnection(Connect.cnst);
        con.Open();
        SqlCommand cmd = new SqlCommand("select [ID],[EmpName] from tblLogin where ID = '" + TextBox1.Text + "'", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        if (ds.Tables[0].Rows.Count == 0)
        {
            ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
            gvdata0.DataSource = ds;
            gvdata0.DataBind();
            int columncount = gvdata.Rows[0].Cells.Count;
            gvdata0.Rows[0].Cells.Clear();
            gvdata0.Rows[0].Cells.Add(new TableCell());
            gvdata0.Rows[0].Cells[0].ColumnSpan = columncount;
            gvdata0.Rows[0].Cells[0].Text = "No Records Found";
        }
        else
        {
            gvdata0.DataSource = ds;
            gvdata0.DataBind();
        }
        con.Close();
    }
    protected void btnPdf_Click(object sender, ImageClickEventArgs e)
    {
        Response.ContentType = "application/pdf";
        Response.AddHeader("content-disposition", "attachment;filename=UserDetails.pdf");
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        //Form.RenderControl(hw);
        gvdata.AllowPaging = false;
        gvdata.DataBind();
        this.BindGridviewData();
        this.BindGridviewData1();
        gvdata.RenderControl(hw);
        gvdata.HeaderRow.Style.Add("width", "15%");
        gvdata.HeaderRow.Style.Add("font-size", "10px");
        gvdata.Style.Add("text-decoration", "none");
        gvdata.Style.Add("font-family", "Arial, Helvetica, sans-serif;");
        gvdata.Style.Add("font-size", "8px");
        gvdata0.RenderControl(hw);
        gvdata0.HeaderRow.Style.Add("width", "15%");
        gvdata0.HeaderRow.Style.Add("font-size", "10px");
        gvdata0.Style.Add("text-decoration", "none");
        gvdata0.Style.Add("font-family", "Arial, Helvetica, sans-serif;");
        gvdata0.Style.Add("font-size", "8px");
        StringReader sr = new StringReader(sw.ToString());
        Document pdfDoc = new Document(PageSize.A2, 7f, 7f, 7f, 0f);
        HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
        PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
        pdfDoc.Open();
        //pdfDoc.NewPage();
        pdfDoc.Add(Chunk.NEWLINE);
      
        htmlparser.Parse(sr);
        pdfDoc.Close();
        Response.Write(pdfDoc);
        Response.End();
    }
}


output