How to show SKU in the packing slip?

SKU is already shown by default in the packing slip, below the product name: https://github.com/wpovernight/woocommerce-pdf-invoices-packing-slips/blob/v2.3.5/templates/Simple/packing-slip.php#L90

If you’re not seeing this please double check that you are using this plugin (WooCommerce PDF Invoices & Packing Slips) and the official “Simple” template.