Although VirtualBox is completely free software (download it for free), many people prefer using the VirtualBox Extension Packs. Despite being an open-source product, some of it's features aren't available in a fully-functional binary format as with a paid version (see "VMware VirtualBox Extension Packs" above). However, all modern smartphones with USB ports can be used in VirtualBox so there is no need to install anything on the computer to use the program. It's very easy to connect your smartphone to your virtual machine - you'll either be prompted on a screen showing that it's working or simply log into your machine from the smartphone. In any case, the program does not require installation of any software on the computer either - it is strictly Reader mode which means everything is basically seen as a PDF file and works cross-platform. In this respect, it follows the similar strategy to Firefox and Chrome for mobile computing.
This means that although there is a VirtualBox installation directory on your computer, it will not be able to access the files / settings that it needs to run. There's no way to create a VirtualBox user folder / installation directory on your computer - so if you want to install the VirtualBox Extension Pack on Windows, follow the instructions at the bottom of this article to move your VDI files onto a PC that can view them. Unfortunately, VirtualBox can not read Microsoft files and so these will not be affected by this issue. You can also move the VirtualBox files across different machines by copying them to a USB drive then booting up your new machine and making use of the USB drive.
There are various other guest additions which you can find inside the VirtualBox download, including some which are actually useful. These include things like allowing you to see what programmes are currently installed on your Windows system, so you can clean out unwanted software that is slowing down your computer or taking up memory. In addition to all these, VirtualBox also has a built in viewer for web pages, which means you don't have to spend time learning HTML to view Microsoft Word documents or PDFs - you'll get it right in an instant.