Erm, what exactly do you mean, mate? Preview will let you select (highlight) text in a PDF if you use the right tool. Are you wanting to literally highlight it and make those sections highlighted for the next person who reads the PDF?
i want a highlighter pen type function. i want to be able to highlight the important parts of an article etc so when i come back to a pdf i can go straight to the important reason i pdf'ed the article in the first place.
i hope there is a free app because the one i remember seeing was $50 or something... too much in my opinion for this simple function. anyway, ive spent too much on apps over the years and i probably dont use 75% of them anymore.
Skim does the trick. Find it easier to use than pdfpen. Now they have added the possibility to export the pdf file with your notes, text markups embedded within the pdf file so all viewers will see them