What PDF software are you using? Most PDF readers have a highlight tool that will allow you to highlight things in the PDF even though you can't change the content. Adobe reader and Fox-it reader both allow highlighting.
I think that it's coming from the Internet. The file is 9 MB but there is no "about" or anything. But they said at the bottom that I can get DC Acrobat Pro for 7 days but I don't think that is going to help me if it's coming from the Internet
Is the pdf file opening inside the browser? Both Firefox and Chrome have extensions for that. Look to see if you have a program such as adobe reader or foxit reader installed which will read pdf files. You will most likely have to change default program for the pdf file extension.
To add annotations or highlights, you may need to save the file and open it in the actual viewer as opposed to the integrated one in the browser. Or if you install a plugin as suggested above that may also work.