The invoice is sent via email, with a copy in the email body and a link to view the invoice on a hosted web page. Your customer can download the invoice as a PDF from there. 

You (the seller) can also download an invoice as a PDF to save a copy for your own records or send to a buyer on request. You can do this by hitting the PDF (Invoice) download button, available on the Invoice Listing page in the Menu drop down alongside each invoice.