We've had this reported on 3 different machines. Dell E7270 laptops with Windows 7, all Dell and Windows updates are recent.
When KB3128008 (Security Update for Microsoft Excel 2013 (KB3128008) 32-Bit Edition) gets applied via updates, then the users Send as PDF function gets broken, generating a "General mail failure. Quit Microsoft Excel, restart the mail system, and try again."
This persists through repairing Office via control panel. If KB3128008 is removed the functionality returns. I'd like to report this to get fixed but not sure how aside from here.
Pictures attached.