https://wordpress.org/support/topic/add-custom-macro/

Add Custom Macro.php

<?php

add_filter( "puiw_printinvoice_create_html_item_row_metas", "my_custom_lineitems_macros", 10, 5);
/* add the macro: {{{remaining-qty}}} */
function my_custom_lineitems_macros ($macros, $item_id, $item, $product_id, $order){
	$product = $item->get_product();
	$current_qty = $product->get_stock_quantity();
	$bought_qty = $item->get_quantity();
	$remaining_qty = (int) $current_qty - (int) $bought_qty;
	$macros["remaining-qty"] = $remaining_qty;
	return $macros;
}