I am new to moodle I am trying to fetch data from a database and converting it in a Pdf File but the pdf file is empty it doesn't display the database elements. I don't get what am I doing wrong, can you help me?

this is the file with the button that triggers the process

<?php
/**
 * @package     tool_report
 * @author      Kristian
 * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

 require_once(__DIR__ . '/../../../config.php');
 defined('MOODLE_INTERNAL') || die();

 $PAGE->set_url(new moodle_url('/admin/tool/report/lib.php'));
 $PAGE->set_context(\context_system::instance());
 $PAGE->set_title(get_string('manage_reports', 'tool_report'));

//what level of the site where are
echo $OUTPUT->header();

?>
<style media="screen">
.btn {
  height: 30px;
  width: 60px;
  background-color: red;
}
</style>

<h1>hi</h1>
<form action="manage.php" method="POST">
  <button class="btn" type="submit" name="button">PDF</button>
</form>

<?php
echo $OUTPUT->footer();

and this is the file that contains the logic:

<?php
/**
 * @package     tool_report
 * @author      Kristian
 * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
 require('fpdf/fpdf.php');
 ob_start(); //
 require_once(__DIR__ . '/../../../config.php');
 defined('MOODLE_INTERNAL') || die();

 $PAGE->set_url(new moodle_url('/admin/tool/report/manage.php'));
 $PAGE->set_context(\context_system::instance());
 $PAGE->set_title(get_string('manage_reports', 'tool_report'));

 //what level of the site where are

 global $DB;

 echo $OUTPUT->header();

$sql = " SELECT *
         FROM mdl_bigbluebuttonbn
       ";
$record = $DB->get_record_sql($sql);
//print_r($record);
$pdf = new FPDF();
$pdf->AddPage();
$pdf->SetFont('Courier','B',16);
$pdf->Cell(50,10,'Course ID', '1', '0', 'C');
$pdf->Cell(50,10,'Course TYPE', '1', '0', 'C');
$pdf->Cell(50,10,'Course COURSE', '1', '0', 'C');
$pdf->Cell(50,10,'Course NAME', '1', '0', 'C');
ob_end_clean();
$pdf->Output();
?><?php
while ($row = mysqli_fetch_assoc($record)) {
   print_r($row); die();
?>
<tr>
  <td><?php echo $row['id'] ?></td>
  <td><?php echo $row['type'] ?></td>
  <td><?php echo $row['course'] ?></td>
  <td><?php echo $row['name'] ?></td>
</tr>
<?php
 }


 echo $OUTPUT->footer();
?>