转换为pdf的代码无法在手机上运行

The following code converts a document into pdf format. The code works fine on laptop but does not work on mobile phone ( On mobile phones a .htm file is downloaded). Please guide. Thanks.

//Generate Pdf
    $pdf = new FPDF('P', 'mm', 'A4');
    $pdf->AddPage();
    $pdf->Image($file_im_pdf . ".jpg", $pdf->GetX(), $pdf->GetY());
    if ($vopt == VOUCHER_OPTION_DOWNLOAD || $vopt == VOUCHER_OPTION_SPDOWNLOAD) {
        $pdf->Output($file_id . ".pdf", "D");
        $return_msg['status'] = "success";
        $return_msg['msg'] = "Downloaded voucher";
    } else {
        $pdf->Output($file_im_pdf . ".pdf", "F");