To help keep track of your incoming purchase orders and their invoices, there's an Invoice received tickbox and an Invoice Number field in the purchase order header where you can enter those details.
You can find these options in the purchase order header by clicking on the blue pencil icon in the top left:

You can use the tickbox to mark that an invoice has been received, and you can enter the Invoice Number into the relevant field.

Frequently Asked Questions
Does the Invoice Number carry over to a Child Purchase Order?
No. If a Purchase Order is only partially received and a Child Purchase Order is created for the remaining items, the Invoice Number will not carry over automatically. This is because the remaining goods are usually shipped separately, with their own invoice. You will need to enter the correct Invoice Number on the Child Purchase Order.