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 records or send it to a buyer on request. You can do this by clicking the "Download PDF" button on Paddle Billing, or "PDF (invoice)" button on Paddle Classic, in the menu drop-down alongside each invoice.