To create a PDF of an invoice go to the Accounting tab -> Invoices. Click on the row of the invoice for which you would like to create a PDF. The invoice will appear. In the sidebar on the right click the "Create PDF" button. The pdf will be downloaded to your computer.
Note: The default view for the invoice table lists unpaid invoices. If you want to create a PDF for a paid invoice you must first select "Paid" invoices in the drop-down menu in the top right corner above the table.