The solution provided by the Quickbooks team does not work - the "attach pdf" box is no longer in that spot. (My account also mysteriously stopped attaching pdfs, very annoying because a number of invoices went out without them).

Policecarman - here's how I figured out how to do it. Open an invoice. Click the gear for settings. Click "change what your customers see". Click "emails". Now you see the "attach pdf" box - select it. It will apply to all invoices going forward. Until it glitches again, sigh.