I am using the fpdf php library to create a PDF file from php

Code:
<?php

require('lib/fpdf.php');

$pdf = new FPDF();
$pdf->AddPage();
$pdf->SetFont('Verdana','B',16);
$pdf->Cell(40,10,'Create this pdf now!');
$pdf->Output("new_pdf_file.pdf", "D");

?>
I get the following error

This page...