Accent in Cell Fpdf function is misplaced “Hélène” vs “Helene”

$pdf->Cell(65 ,3, "Helene",0, 1, 'R');
$pdf->Cell(65 ,3, "Hélène",0, 1, 'R');
$pdf->Ln(2);

gives 2 results differents :
see result : https://www.sitform1.ifra-gm.org/outils/cartes_visites/test.pdf

Why accents provoque this problem ?
Thanks for your answer.

Regards