HTML2PDF include meta, links css and js

I'm trying to create a pdf with the html2pdf library, but it says "headers already sent" when I add any of the following lines:

<meta http-equiv="Content-Type" content="text/html;charset=ISO-8859-1" />
<link href="css/style_vista.css" rel="stylesheet" type="text/css" />
<script src="js/jquery/ver/1.7.2/jquery.min.js" type="text/javascript"> 
</script>

I build the pdf in the following way pdf.php is the html:

    ob_start(); 
    include 'pdf.php';
    $content = ob_get_clean();       
    try
    {
        $html2pdf = new HTML2PDF('P', 'FOLIO', 'es', true, 'UTF-8',array(10, 20, 10, 10));//array(izq,arr,der,aba)
        $html2pdf->pdf->SetDisplayMode('fullpage');
        $html2pdf->writeHTML($content);
        $html2pdf->Output('recepcion_vista.pdf');
    }
    catch(HTML2PDF_exception $e) {
        echo $e;
        exit;
    }
    ob_end_flush();  

Any help is welcome.