I can highlight text in Preview and if I had Adobe Reader, I'm sure I could do it there too. What app are you using and where did the PDFs come from? Depending on how the PDF is made, some won't allow you to highlight their text because it was inserted as an image.
I think he means highlight the text in a different colour, not selecting the text. I'm looking for a solution to this too, as I read a lot of journal articles and would like to highlight relevant sections.
You really need Adobe Acrobat (the full version) to do this properly, if I remember correctly.
BTW, friendly reminder:
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