Using older versions of Outlook for Mac for personal email and calendar
I started having this issue yesterday, also on v16.63 beta (and coming/going thru Google), and based on EXCELLENT comments from many folks here, I quickly (OK, it took a while) switched back to Legacy version. Once there, couldn't drag-n-drop PDF file into email, but was able to Attach File by navigating to it and Choosing it... and it seems to have worked! I'll try DnD again later...
Agree with everyone else's B&M's here directed at Microsoft - C'MON MS PEOPLE, get on the ball and fix this ****!