You don't need to buy an expensive program to add comments or notes to a PDF. Here's how to use Apple's built-in application, Preview, to do.
PDF Annotation: adding notes and comments to a PDF document. mlbinternational.come lets you easily add notes and comments to PDFs (and other types of document.
In this comprehensive guide, learn how to add a comment to a PDF file, including comments, drawing, tools, stamps, and more using Acrobat.