Go to your administrator screen. The URL looks like this: https://app.smartkeyword.io/c/XXXX, and click on "account and team settings":

You will find all your invoices below the users of your account:

All you have to do is to click on the invoice reference number to have access to the whole document:

You just have to click on the button "download the document" to save your invoice on your computer. 😊