Got error message "the file is unsupported..." when I tried to open a pdf I had just closed. Why?

I have a set of margery allingham books in pdf.  So far I've read 3 of them in Adobe Reader.  I just ended and closed the third one and then decided to open it again to look at something I wanted to re-read.  It wouldn't open--I got the error message "this file is unsupported or the pdf is damaged."  This about a file I had just closed 2 minutes before.

I use a macbook pro, osx yosemite

What is going on?  Please fix this glitch!