You can generate an invoice with the 'PDF A4' button in the Payment section of a sale, an use your A4 printer to print it.

As long as the sale is not closed, the 'Proforma' (draft) mention will be printed on the PDF document.
You must closed the sale to print a valid document with an invoice number.

You must linked the sale to a customer AND an invoice address, to print the 'Invoice' mention on the PDF document, otherwise it will be a 'Receipt'.
When a customer is assigned to a sale, the invoice and delivery addresses are automatically selected with the default customer address.

To configure your invoice layout, go to 'Settings/Printing/Invoice PDF A4'. More information.

You can also create several custom templates for your PDF documents (invoices, quotations, purchases orders…). More information.