To get started, log into your Nimbus account by following the link here.
If you need to reset your password you can do it via the link here.
Once logged in, click on Billing > My Invoices:
Then click on the invoice number for the invoice you would like a PDF copy of:
The invoice will then open in your browser. Click the "Download" button in the bottom right hand side of the page to save a copy of the invoice to your computer: