You are able to download a PDF version of an invoice while the invoice is in any state (Draft, Outstanding, Overdue, or Paid). To do this, navigate into the invoice page itself, click on the three dots on the top right of the screen and click on "Download PDF".
Alternatively, you can also download a version of the PDF by previewing the Invoice itself and clicking on the "Download PDF" button on the bottom of the invoice. Clients will also be able to download the Invoice as a PDF from the same place:
Invoice receipts included on the downloaded PDF
Invoices which have been paid will also include documentation of the payment towards the bottom of the invoice itself. Once an invoice has been paid, the recorded payment will be noted on the last page of the invoice, available for both you and your client's records:
A link to download the invoice will be emailed to the client once the invoice has been marked as 'Paid'.