Preview on the mac has a tool menu that contains annotation tools. These tools allow you to easily open any pdf file and "mark it up." These tools are quick to use, giving you easy access to editing and emphasizing a document. PDF files can be created from anything that is printable. You can save your work as a PDF, then display the PDF files on your board or screen with a projector. Then, using their tools, you can edit, highlight changes or re-emphasize strong parts of the document. There are many uses for this in the classroom.