Erreur html 2 pdf

Bonjour,

je veux gernerer une page pdf grace a html 2 pdf je ne sait pas pour quoi mon code affiche l'erreur suivant :
TCPDF ERROR: Some data has already been output, can't send PDF file

<?php

 require ('mysql_db_connect.php');

 ob_start();


?>
 <page backtop="5%" backbottom="5%" backleft="5%" backright="5%"> 
     <table style="width:100%">
  <tr>
   <th>Numéro</th>
   <th>État</th>
   <th>Affectation</th>
   <th>Responsable</th>
   <th>Brève description</th>
   <th>Code Priorité</th>
   <th>Date/Heure d'ouverture</th>
   <th>Date/Heure de clôture</th>
  </tr>
   <?php
   $que = "SELECT * FROM `table 1` WHERE ((`Brève description` Like '%opm%' Or `Brève description` Like '%oracl%' Or `Brève description` Like '%gmao%' Or `Brève description` Like '%opm%' Or `Brève description` Like '%saisi%' Or `Brève description` Like '%firewal%' Or `Brève description` Like '%instrumen%' Or `Brève description` Like '%tonnage%')) OR ((`Brève description` Like '%export%' And `Brève description` Like '%pdf%')) OR ((`Brève description` Like '%formule%' Or `Brève description` Like '%bascule%' Or `Brève description` Like '%opm%' Or `Brève description` Like '%capteur%')) OR ((`Brève description` Like '%étalonnage%' Or `Brève description` Like '%compteur%' Or `Brève description` Like '%remontée%')) OR ((`Brève description` Like '%débimètre%' Or `Brève description` Like '%automate%' Or `Brève description` Like '% tag %' Or `Brève description` Like '%jinitiator%')) OR ((`Brève description` Like '%capteur%' Or `Brève description` Like '%forçage%' Or `Brève description` Like ' AP ' Or `Brève description` Like ' FA ' Or `Brève description` Like ' GTA ')) OR ((`Brève description` Like '%écart%' Or `Brève description` Like '%coefficient%' Or `Brève description` Like '%GPAO%' Or `Brève description` Like ' INV ' Or `Brève description` Like ' PO ' Or `Brève description` Like ' AR ')) OR ((`Brève description` Like 'formation%' Or `Brève description` Like '%débimètre%' Or `Brève description` Like '%EAM%' Or `Brève description` Like '%instrument%')) OR ((('Ou') Like '%rapport%')) OR ((`Brève description` Like '%opc%'))LIMIT $start,$end";
   $result =mysql_query($que);
   
   while ($row = mysql_fetch_array($result) ) {
   ?>
   <tr>
   
   <td><?php echo $row['Numéro'];  ?></td>
   <td><?php echo $row['État']  ;?></td>
   <td><?php echo $row['Affectation'];  ?></td>
   <td><?php echo $row['Responsable'] ; ?></td>
   <td><?php echo utf8_encode($row['Brève description'] ); ?></td>
   <td><?php echo $row['Code Priorité']  ;?></td>
   <td><?php echo $row['Date/Heure d\'ouverture']; ?></td>
   <td><?php echo $row['Date/Heure de clôture'] ;?></td>
   
   </tr>
   
  <?php
   }
  ?>
     </table>
    
 </page> 

 <?php
 $content = ob_get_clean();
 
 require('html2pdf/html2pdf.class.php');
 
    $pdf = new HTML2PDF('P','A4','fr','true','UTF-8');
    $pdf->WriteHTML($content);
    $pdf->Output();
 ?>
 

merci