To finish my work, I need to add text to a PDF on Mac. How can I do this?
Assuming you're using Moutain Lion, you'll find instructions on how to do this here: http://macintoshhowto.com/software/how-to-edit-a-pdf-document-on-a-mac.html .
This is not a particularly elegant solution but if the pdf is locked and you cant edit it, you could try the following.
Highlight the entire pdf document. Copy and paste it into a word document. Add your additional text, then save the word doc back into a pdf. On a Mac, do this by going File - Print - then hit the PDF option on the bottom left hand corner of the print menu.