PDF files are most commonly viewed with Acrobat, a free program made by Adobe, which acts as either a plug-in for your web browser or as a stand alone application and is available for a wide range of standard and esoteric operating systems. PDF files allow documents to be published in a form where they look identical on any platform-Mac, Windows, UNIX, OS/2, etc. The advantage of this is clear when you realize that HTML documents (like this page) can look totally different on a Mac as compared to Windows. PDF documents allow us the ease of mind of knowing that when you download our preparation sheets and print them out in PDF format, we know exactly how they are going to look.