The account admin can access the billing portal here to download your account's invoices.

Navigate to your billing portal in the usual manner, by clicking your user icon in the upper right hand corner of your screen, then clicking "Billing".

Alternatively, this may also be accessed through the sidebar:

Within your billing portal, you'll see all of your historical invoices under the "Invoices" tab, to the right hand side of your screen. Click any invoice number to download a PDF version of that invoice.