Ah ha! I may have sussed the problem.
Are you using Win 10 and it's Edge browser Paul?
I just had the same problem on my Win 10 machine.
(It's new. Still larnin' to drive it, so it don't get used much yet)
Installing Adobe Reader and setting it as default for .pdo has cured the problem.
Windows Edge has still got some larnin' to do, I think.
