How to add a custom field?

Hello!

For that you will need the full version and use custom fields: https://wooinvoice.rednao.com/knowledge-base/integrating-the-pdf-builder-with-other-woocommerce-plugins/

Regards!

with the full version I will be able to add for example this: billing_wooccm11 ?
(is a custom checkout field) that i need in the invoice.
Thks

Yes, the custom field wizard most likely will be able to find and create the custom field that you need (as explained in the tutorial) but if not you can create a support ticket to get assistance to create the custom field.

Regards!