Yes, you must have Mathematica on your computer before you can even install the Handbook.
The Handbook will work on Mathematica versions 9 and higher, including the current version.
Yes, you can buy your activation code (proof-of-purchase) from any machine, and the activation code will be sent to you in an email. You can download the installer and install the Handbook on any other machine which can run Mathematica. Once you have installed the Handbook, your activation code is linked to the computer on which you installed it. You can re-install updates of the Handbook on the original computer using the same activation code, but you cannot use the activation code to install another copy of the Handbook on a different computer.
Launch Mathematica. Click on the Help button on the tool bar at the top. A menu drops down. Select “Documentation Center” A window will open. Go to the bottom left and select “Add-ons & Packages” A new window will open. Look for “MathematicaHandbook” and click on it. A blank browser window will open with a list of headings. Click to navigate to the notebook you want to read.
I noticed that there is a new release of the MathematicaHandbook on the website. Do I need to pay again?
Your current version will continue to run for as long as you have it on your machine. Your current activation code may be used to download updated releases for 3 years from the time you received it. After that time, you will need to use another activation code to get updated versions.
The Handbook will work on Windows 9 and higher and OS X 10 and higher.