Adobe Acrobat could not open...because it is either not a supported file type or because etc

I see forms of this being addressed in other posts but not one that has been answered as far as I can see.

I use Acrobat Pro DC to compile a series of screenshots into PDF format.  I then try and fail to open the file in Acrobat Pro DC...I get the error message "Adobe Acrobat could not open...because it is either not a supported file type or because etc"...

But the file opens fine in Mac Preview!

What's up?

I did notice that in the process of creating the pdf, there's a progress bar at the bottom that never goes past "checking security" or something like that...but then the file's created.