It means that you can not highlight a PDF file directly. PDF format is a complete format that can not be edited.
And due to its high encryption standard, PDF owners can protect their PDF information by adding PDF passwords or other using permissions. The PDF format appears consistently on different computers with different operating systems. When sharing information between multiple people on different computers, using a PDF file ensures that everyone is looking at the same thing. 'I am wondering whether it is possible to highlight the text in a PDF document while you are reading it with the Adobe Reader 7 or not? And also save the highlight? If not, then do you know any other programs that can open a PDF file, highlight, and save the change?' - From PDF Users
