chiedo scusa. E' giusto scrivere

$this->SetMargins(100,100);

e dove lo devo inserire?

...

class PDF extends FPDF
{
function PDF($orientation='L',$unit='mm',$format='A4')
{ include 'function/function_Pdf.inc.php'; }
function Header()
{ include 'function/function_Header.inc.php'; }
function Footer()
{ include 'function/function_Footer.inc.php'; }
}
$pdf=new PDF();
$pdf->AddPage();

//*********************************************
INIZIO DOCUMENTO
//*********************************************