public function downloadInvoice()
{
    $id  = $this->uri->segment(2);
    $where = array('invoice_id'=>$id);

    $data['invoice'] = $this->Common_model->getDataWhere("invoices","*",$where);

    $mpdf = new MpdfMpdf();
    $this->session->set_userdata($data);
    $filename = $invoice['invoice_no'].'.pdf';
    header("Content-Disposition", "attachment; filename='.$filename.'");
    $html=$this->load->view('user/pdf/download_invoice',[],true);
    $mpdf->WriteHTML($html);
    $mpdf->Output($filename, 'D');
}