foreach loop print only one record from MySQL table

I am trying to print records of a table into a pdf file using fpdf, to this I am using for each loop, but it just prints one record though it works fine without fpdf class. here is the code:

if(isset($_GET['show_id'])){
    $id = ($_GET['show_id']);
    $comments = selectAll('proposal_comment', ['proposal_id' => $id]);

    foreach($comments as $comment){
        $pdf = new FPDF();
        $pdf->AddPage();
        $pdf->SetFont('Arial','B',16);
        $pdf->Cell(100,10,$comment['message']);
        $pdf->Output();
    }
}

any idea why is it like this?