How to attach PDF to email using Swiftmailer in Symfony2

Question

I am sending an email using swiftmailer in symfony2, but I would like to add a specified PDF file as a file attachment to the email. How would I do that?

Here is my current code:

$message = \Swift_Message::newInstance()
    ->setSubject('Hello Email')
    ->setFrom('[email protected]')
    ->setTo('[email protected]')
    ->setBody(
        $this->renderView(
            'HelloBundle:Hello:email.txt.twig',
            array('name' => $name)
        )
    )
;
$this->get('mailer')->send($message);