This started happening to me yesterday.  My invoices have always attached as PDFs to my emails.  Now, all of a sudden on a Monday morning, I'm getting all of these messages from my customers that there are no attached PDF invoices on their invoices.  And I need to go back in, and re-send all of my weekly invoices.  I was thankfully able to fix the problem by using the information provided by Support by going into the Settings and selecting the PDF attach button.  (The email invoice template already had it checked even though it wasn't happening.) However, I didn't do anything to make this happen!  This must have been some sort of glitch in an update or new coding or something.  It's extremely frustrating and poor customer service to make changes like this to customers with no notification.  I should not have had to go into the Settings and fix settings that always worked in the past.