You are probably accessing sites with articles that have been written to pdf. format, which is a means by which they can not be altered by viewers keeping them true to the author's intent. There are many tutorials here on the board in pdf. format, too, and you can't access them without having Adobe Reader on your computer....at least the free addition. You can download that here for free:
http://www.adobe.com/products/reader.html
Jan in VA