I use the “WooCommerce PDF Invoices & Packing Slips” plugin, available on the WP Repository, to invoice my clients. At times, for accounting purposes, I need to show the VAT/TAX ID of the client on the PDF invoice.
So, I open the order via WooCommerce > Orders, open the order I want to edit, add a “Note to the Customer” from the right hand side… and then have the following snippet in my functions.php adding such information on the PDF invoice!