There are three exports on a work order, and they sit together in the same menu:
Export as work order is the PDF you already use — it was previously called Export as pdf on the list and Download PDF on the work order. Only the name has changed.
Go to Work orders.
Open the ⋮ menu at the end of a row — or open the work order and use its Actions menu.
Choose Export as quote or Export as invoice.
Check the details in the form that opens, enter your document number, and confirm.
From the Work orders list
From the work order details page
You can also export while you are looking at a work order. Open it, select Actions in the top right, and choose Export as quote or Export as invoice. The same form opens and the PDF downloads the same way.
You can export at any work order status, and as many times as you need. A quote before the work is approved and an invoice afterwards are two separate exports of the same work order — one does not turn into the other.