In order to use the "additional" fields feature, the premium version of the plugin is required. Check the simple version otherwise: https://gist.github.com/mircobabini/64ead066b22575f6948d133c82500555

wc-wpo-wcpdf-billing-address-company-vat-advanced.php

<?php
add_action( 'wpo_wcpdf_billing_address', function ( $billing_address, $invoice ) {
	if ( ! empty( $invoice ) ) {
		$customer_type = $invoice->order->get_meta( '_additional_wooccm0', true, 'view' );

		if ( 'Azienda' === $customer_type ) {
			$company_name = $invoice->order->get_meta( '_additional_wooccm1', true, 'view' );
			$company_vat  = $invoice->order->get_meta( '_additional_wooccm2', true, 'view' );

			$billing_address_prefix = "";
			$billing_address_prefix .= "<b>$company_name</b><br>";
			$billing_address_prefix .= "<b>PIVA: $company_vat</b><br><hr>";

			$billing_address = $billing_address_prefix . $billing_address;
		}

		// $billing_address .= ' //';
		// $billing_address .= $order->get_field( 'billing_first_name');
	}

	return $billing_address;
}, 10, 2 );