en fait je fait ca:
ob_start();
echo $finalhtml;
$content = ob_get_clean();

// conversion HTML => PDF
require_once('html2pdf_v322a\html2pdf.class.php');
$html2pdf = new HTML2PDF('P','A4','fr');
$html2pdf->pdf->SetDisplayMode('fullpage');
$html2pdf->WriteHTML($content);
$html2pdf->Output($outputfile,'F');

mais si je fait ca c pareil:
// récupération du contenu HTML
// conversion HTML => PDF
require_once('html2pdf_v322a\html2pdf.class.php');
$html2pdf = new HTML2PDF('P','A4','fr');
$html2pdf->pdf->SetDisplayMode('fullpage');
$html2pdf->WriteHTML($finalhtml);
$html2pdf->Output($outputfile,'F');