You will need Acrobat Reader to open the files. If you don't have this utility, you can download and install it from the "Downloads" page of the website, or by clicking the logo on the left. Don't hesitate to install Acrobat Reader. It's free, and the publisher (Adobe) will periodically inform you of available updates in an unobtrusive way. No annoying e-mails, no commercial banners, no pop-ups.