Export Sharepoint list to PDF for all pages

I am using the below code to export the list to PDF. However my list has different pages and this code only works for the current page. How can I modify this to export list items of all pages.

private DataTable SearchItem()
        {
            DataTable dt = new DataTable();
            SPSecurity.RunWithElevatedPrivileges(delegate
            {
                SPList list = SPContext.Current.Web.Lists["Spreadsheet list"];
                string query = null;
                SPQuery qry = new SPQuery(list.DefaultView);
                //Set export for a particular view
                //Guid guidViewID = list.Views["View 1"].ID;
                //SPQuery qry = new SPQuery(list.Views[guidViewID]);
                int iCnt = 0;
                try
                {
                    qry.ViewFields = "<FieldRef Name='Definition' /><FieldRef Name='Comments' />";
                    qry.Query = query;
                    qry.ViewFieldsOnly = true;
                    dt = list.GetItems(qry).GetDataTable();
                }
                catch (Exception ex) { Response.Write(ex.Message); }
            });
            return dt;
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            DataTable dt1 = SearchItem();
            //Get the data from database into datatable
            GridView GridView1 = new GridView();
            GridView1.AllowPaging = false;
            GridView1.DataSource = dt1;
            GridView1.DataBind();
            Response.ContentType = "application/pdf";
            Response.AddHeader("content-disposition","attachment;filename=DataTable.pdf");
            Response.Cache.SetCacheability(HttpCacheability.NoCache);
            StringWriter sw = new StringWriter();
            HtmlTextWriter hw = new HtmlTextWriter(sw);
            GridView1.RenderControl(hw);
            StringReader sr = new StringReader(sw.ToString());
            Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);
            HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
            PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
            pdfDoc.Open();
            htmlparser.Parse(sr);
            pdfDoc.Close();
            Response.Write(pdfDoc);
            Response.End(); 
        }