Invoice is created after an order has been fulfilled. That means, a sales order has been shipped and a tracking number has been assigned to the fulfillment.
To view invoices:
Go to "Invoices" from the menu.
You'll see all the invoices and their status. On the actions column of the invoice you want to view, click "View".
After clicking "View", you'll see the details of the invoice and you have an option to download it in PDF format.