PDF files are files that require a PDF viewer to read. Adobe Acrobat Reader is a very common free program that most users already have installed. GET ACROBAT READER

ZIP files are compressed files that take up less storage space but must be un-zipped before you can use them. They are commonly used over the internet to save on web space. Most users will already have an un-zip program installed. (Windows XP has the feature built in) If you need to install an un-zip program, they are free and many versions from different companies are available. Winzip is a common choice. You should not need to register (pay for) any unzip program unless you plan on using it to compress your own files.