Get Attribute in PDF

Hi, our client wants to display product attribute, because that is how they identify their products. How can I include it? I found some code on other forum:

// Add product custom code

function product_custom_code( $product ) {
$sifra = $product->get_attribute(‘pa_sifra’);
echo $sifra;
}
add_action( ‘wcdn_order_item_after’, ‘product_custom_code’ );

But this does nothing, no attribute is shown. I want to display it just below SKU.

Please help?