Just follow the steps below to send invoices as PDFs. Just to make sure it works you can send one test invoice to yourself.
Select the Gear (⚙) Icon in the top right-hand corner > Your Company > Account and Settings > Sales > Online Delivery > Attach Invoice as pdf > Save.
With this change in setting, you should now be able to send invoices to your customers with PDFs attached.
See screenshot below for reference.