If you plan to annotate on a Mac and don't want to download another program like Adobe Reader, you can use the built-in Preview app that comes standard with all Macs. To use the program simply open your pdf. View the annotation options by navigating to Tools > Annotate and select the option you would like to use.
You can also show the markup toolbar at the top by selecting View > Show Markup Toolbar.
Highlight, underline, draw and add notes to the document. Once you are done, make sure to save.
Changes to the document should be visible in other programs like Adobe Reader.