SOLVED for Presta 1.5 with 2 files, thanks
if ($template == 'es/order_conf') {
$file1 = _PS_ROOT_DIR_ . '/TerminosyCondiciones.pdf';
$file2 = _PS_ROOT_DIR_ . '/FormularioDesistimiento.pdf';
$message->attach(new Swift_Message_Attachment(file_get_contents($file1), 'TerminosyCondiciones.pdf', 'application/pdf'));
$message->attach(new Swift_Message_Attachment(file_get_contents($file2), 'FormularioDesistimiento.pdf', 'application/pdf'));
}