FPDF：填写表格pdf并防止将来修改

I have a form pdf that I fill out and save with the Form Filling Script from FPDF

<?php

/***************************
  Sample using a PHP array
****************************/

require('fpdm.php');

$fields = array(
    'name'    => 'My Name',
    'datum'   => 'Berlin, 23 February 2017'
);
$oldmask = umask(0);
$pdf = new FPDM('template.pdf');
$pdf->Load($fields, true); // second parameter: false if field values are in ISO-8859-1, true if UTF-8
$pdf->Merge();
$pdf->Output('test.pdf');
umask($oldmask);
?>

The script works, but the output test.pdf is still editable. How can I prevent future modifications of the document by the user?