Adobe Acrobat Reader is the free standard for Portable Document Format (PDF) file viewing. Many programs install this if you don't already have it, as PDFs have become the main standard for digital manuals and other reference documents - so having it pre-installed on your computer can save time later on when you need to access such files or load other programs. There are also open-source competitors to Reader, if you prefer that approach, but they may not be able to view all PDF files properly.