YITH WooCommerce PDF Invoice and Shipping List allows you to automatically generate the invoice in PDF format for each registered order in your shop.
The combination with YITH WooCommerce Checkout Manager allows you to also edit the fields (VAT and SSN) that the PDF invoice plugin adds to the checkout.
Just make sure you have enabled and activated both plugins to make the integration work. No other action from your side is required.
To read more about the settings of YITH PDF Invoice, please, refer to the official plugin documentation.