An MPP Viewer is simply a software application that allows you to open and view Microsoft Project Plan files. Microsoft Project is a Project Management Software program, developed by Microsoft. It’s basically designed to assist project managers with effective project management solutions. However, Microsoft itself does not provide any Viewer application for its Microsoft Project (.mpp) files. And sometimes it might happen that you receive a file developed in Microsoft Project (for evaluation purpose, lets say), but you are not able to open it since you don’t have Microsoft Project installed on your system. This is where MPP Viewers comes handy. They lets you easily open and view Microsoft Project Plan files.