Hi @saniat,
This can be done without any code using a custom block within the customizer, included in the Premium Templates extension, or adding some PHP code, using our PDF template action hooks.
If you choose the first option, you just need to add a custom block with these settings:

In the case that you prefer to try with code, add this code snippet to your site:
/**
* Show the VAT number on the invoice
*/
add_action( 'wpo_wcpdf_after_billing_address', function($template_type, $order){
if( $template_type == 'invoice') {
if ( $vat_number = $order->get_meta( 'vat_number' ) ){
echo '<div>VAT: ' . $vat_number . '</div>';
}
}
}, 10, 2 );
If you haven’t worked with code snippets (actions/filters) or functions.php before, read this guide: How to use filters.
Let me know if you manage to display the VAT number in your invoices!