In WooCommerce, click on WooCommerce, and then Booster Settings on the left. In the first row under the heading tabs, click PDF INVOICING & PACKING SLIPS. In the second row under that, click “Templates”. Scroll down to the template for the document you want to add the custom field to. Add the following where you want the field to appear, but replace “billing_wcj_checkout_field_1” with the name of your custom field:
[wcj_order_checkout_field field_id="billing_wcj_checkout_field_1" before="Buyer PO Number:"]
I hope this helps!