functions.php

add_filter( 'wp_wc_invoice_pdf_after_billing_address', 'woocommerce_email_order_meta_add_email_to_invoice_pdf' );

function woocommerce_email_order_meta_add_email_to_invoice_pdf($order) {
	
	if ( method_exists( $order, 'get_billing_email' ) ) {

		$customer_email = $order->get_billing_email();

		if ( ! empty( $customer_email ) ) {

			echo "<br>" . $customer_email;

		}

	}
}