Preview is an excellent program for viewing PDF files and images in most any format. There are two features that few of us know about: annotating and marking-up PDFs. It’s a lot like the Track Changes function in Pages and other word processors, and is a great way to share comments with a group working on a project.
In Preview’s Tools menu you’ll see Mark Up and Annote. Mark Up lets up do just that: strike through, highlight, or underline text; Annotate lets you append notes and web addresses in the margins.
In typical clean and usable fashion, Apple added keyboard shortcuts for these powerful tools. You can also add buttons to Preview’s toolbar by right-clicking on it and selecting Customize.
Preview is more powerful than many seem to think, and it’s vastly superior to Adobe Reader for everyday PDF consumption, if only because it launches almost instantaneously.