Thanks to Mark, once again. Oddly enough, when I saw the post re. the problem, I was on my android device and downloaded it to that. There was a sort of hiccup and it seemed as if it hadn't downloaded. Within seconds I hit download once more and away it went. Perfect Viewer then opened the file and I read the first dozen pages. Glad it's all serene now.
This isn't one of my own scans, it's a file I found tucked away in an internet backwater and when I downloaded it from there to my own pc it was fine and CDisplay opened it perfectly.