We implemented a barcode on the PDF in FireGento_Pdf. In this case, the barcode represents the order number. But you could have a look at the code to see how it is done here. The main part is this:
$barcodeConfig = array(
'drawText' => false,
'orientation' => 90,
'text' => $order->getIncrementId()
);
$rendererConfig = array(
'verticalPosition' => 'middle',
'moduleSize' => 0.9
);
// create dummy Zend_Pdf object, which just stores the current page, so that we can pass it in
// Zend_Barcode_Renderer_Pdf->setResource()
$pdf = new Zend_Pdf();
$pdf->pages[] = $page;
/** @var $renderer Zend_Barcode_Renderer_Pdf */
$renderer = Zend_Barcode::factory('code128', 'pdf', $barcodeConfig, $rendererConfig)->setResource($pdf, 0);
// calculate left offset so that barcode is printed on the right with a little margin
$leftOffset = $page->getWidth() - $renderer->getBarcode()->getWidth(true) * $renderer->getModuleSize() - 10;
$renderer->setLeftOffset($leftOffset);
$renderer->draw();
Mage_Sales_Model_Order_Pdf_Packaging
is inapp/code/core/mage/sales/model/order/pdf/packaging
.