Ma perché definisci il metodo PDF?

Fai semplicemente così:

<?php
require ( "fpdf.php" );

$pdf=new FPDF();
$pdf->AddPage();
$pdf->SetMargins(100,100);
$pdf->SetFont('Arial','',8);
$pdf->Cell(100,4,'ABC','LB',0,1,'C');
$pdf->Output();
?>