Importing PDF to Illustrator, fonts displayed as boxes with and X

I read here about how the PDF isn’t communicating the characters to Illustrator, so you can’t just change the font – thus the X-in-a-box issue. That made me think of how to ensure the PDF kept the characters. I tried saving as PDF A and then opened in Illustrator and I got the substitution warning window, all the words there.

https://community.adobe.com/questions-652/importing-pdf-to-illustrator-fonts-displayed-as-boxes-with-and-x-788009