Print an invoice
Login and go to Sales > Invoices. Filter or otherwise find the one you'd like to print.
Click on the invoice and you'll see a nice representation of the final invoice that is ready to be sent/was sent to your customer.
Click on the "..." dropdown at the top right of the invoice, and select Print.
Your browser will download a PDF copy of the invoice, formatted for print. Open that up (it may be in your Downloads folder or wherever your browser usually stores downloads) and print from your PDF viewer of choice.