The PDF file has become something of a standard file type, particularly when it comes to distributing documents via the internet. This portable format can be used on a variety of operating systems and each document will look exactly the same on each computer.
Adobe Reader is one of several programs that can be used to view PDF files and provides an easy way to navigate long documents. A search facility makes is possible to jump to instances of a specific word or phrase, while support for hyperlinks, both internal and external, means that you can jump to different sections of a document, or view related information online.
Where the author has allowed it, PDF documents can be printed out, with the guarantee that what you see on screen is what you will see on paper. Support for Adobe Flash means that feature rich interactive documents can be viewed as well, and audio and video formats such as MP3 and QuickTime can also be embedded in PDFs.
As well as simply viewing files, documents can be read aloud, and shared online at the Acrobat.com web site. This is not a lightweight program, and it can suffer from being a little sluggish, but for unrivalled support for the PDF format, this is the program to go for.
Note that this is Adobe Reader for your mobile device - iPhone, iPad, Android.
Unrivalled PDF supported in a slightly bloated program, but no computer is complete without a copy of Adobe Reader.