I've had plenty of success with using a form that submits to a php script that uses fpdf and passes in the form fields as get variables (maybe not a great best-practice, but it works).
AddPage();
$pdf->SetY(30);
$pdf->SetX(100);
$pdf->MultiCell(10,4,$_POST['content'],0,'J');
$pdf->Output();
?>
and the you could have something like this.
This skeletal example ought to help ya get started.