Send attachments with PHP Mail()?

I need to send a pdf with mail, is it possible?

$to = "xxx";
$subject = "Subject" ;
$message="Example message with <b>html</b>";
$headers="MIME-Version: 1.0" . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$headers .= 'From: xxx <xxx>' . "\r\n";
mail($to,$subject,$message,$headers);

What am I missing?