Try it on the other document and see if you get the same result.
Relaunch the application and try to perform it again.
Does the problem occur with a particular PDF document? What happens when you try to highlight text in different PDF documents?
Would you please long press (press & hold) on a word to see if you can actually select text?
Your PDF document may be a scanned PDF (meaning that it only contains images of text, not real text to highlight).
Please let us know if you would like us to take a quick look at your PDF document to diagnose the cause of the problem.