我需要将客户填写的表格细节以pdf格式作为附件发送给店主，它不会用php mail（）方法发送pdf，它可以用phpmailer（）正常工作

require_once("dompdf_config.inc.php");
    $dompdf = new DOMPDF();
    $dompdf->load_html($message1);
    $dompdf->render();
//$dompdf->stream("sample.pdf");exit;
    $pdf = $dompdf->output();
    $attachment = chunk_split(base64_encode($pdf));
    $filename = "vendor.pdf";

$mail = new Mail($config->get('config_mail'));
    //$mail = new Mail();
    $mail->addStringAttachment($pdf, $filename, $encoding = 'base64', $type = 'application/pdf', $disposition = 'attachment');
    $mail->setTo('owneremail@yahoo.com');
    $mail->setFrom($config->get('config_email'));
    //$mail->setFrom('info@mdpocket.com');
    $mail->setSender($config->get('config_name'));
    $mail->setSubject('Vendor Form');
    $mail->setText("Find Attached Files");
    $mail->addAttachment($path_of_uploaded_file);
    $mail->send();

addStringAttachment（）函数在mail（）方法中不起作用。我如何发送PDF文件？