I’m afraid this is not something we are planning to implement, to quote a similar post:
I’m afraid we won’t be adding this as a system-wide option. There are many good reasons not to use PDF attachments and this feature was only ever added as a convenience for those end-user clients who insisted on having their invoices in PDF.
The web based invoice takes one click to view and can be tracked, it will also work seamlessly on any desktop or mobile device. Clients can still download a PDF version from the web preview.