PDF with RockPdf

[Frank Schneider]

I try to use RockPdf

when i dump $pdf->dump(); the pdf show perfect

When i $pdf->download('pdf-filename.pdf'); it not work

It show it is no valid pdf

$pdf = $modules->get('RockPdf');

$pdf->set('SetHeader', 'This is my header text');

$pdf->write('Hello World ' . date('H:i:s'));

$pdf->write('<!-- my custom comment -->');

$pdf->html(); // output html in tracy console

$pdf->save(); // generate pdf

//$pdf->dump(); // dump pdf to iframe in tracy console

$pdf->download('pdf-filename.pdf');

What is the reason