Thanks HTH. I just realised myself also, that I was trying to highlight bits of a pdf that was an html, which is why it didn't work. Thanks for the other advice. I'll give it ago.

Why can you highlight some text on pdf documents but not on others? When you select highlight and hover over the text, sometimes you get a square, which you know will enable you to highlight. Other times on other documents, the square doesn't appear and highlighting doesn't work. Any ideas?