Adobe Reader has become the standard for reading ebooks and documentation online. For Windows, there are 3rd party readers like FoxIt that can be substituted, but on Linux, the best choice is still Adobe Reader.
Before you install, you will want to make sure that you have selected the Multiverse repository.
To install just Adobe Reader, open up a terminal window and type in the following command:
sudo apt-get install acroread
If you want to install Adobe Reader as well as the plug-in for Firefox, then use this line instead:
sudo apt-get install acroread mozilla-acroread acroread-plugins
If you are reading this in Firefox, you may want to restart Firefox for the plug-in to start working.