$FileName = "sampleName.pdf";
require_once 'vendor/autoload.php';
use DompdfDompdf;
$dompdf = new Dompdf();
$dompdf->loadHtml($str);
$dompdf->setPaper('letter', 'landscape');
$dompdf->render();
$pdf = $dompdf->output();
if (file_put_contents($FileName, $pdf) !== FALSE) {
    $dompdf->stream($FileName, array('Attachment'=>1));
} else {
    echo "Error: PDF was not generated.";
}