You can access the invoices generated by an order in two ways.

From the list of orders:

Access the menu at the end of a line corresponding to an order. If invoices have been generated for this order, you are offered the option “Associated invoices”. Click this option to display the list of invoices that are associated with the order and have been sent.

From the order details view:

If invoices have been generated for this order, there will be a link to them at the top right of the order you are viewing.

You click this link to access the list of invoices that are associated with the order and have been sent.