For almost as long as there's been Word, Excel and PowerPoint, Microsoft has made free viewer apps available so that you can view documents even if you don't have a copy of the application that created them. Not that this is particularly important nowadays, of course, because there are plenty of free office suites that can open, and indeed edit, native Microsoft Office files.
But in the case of .MPP files, as produced by Microsoft Project, it's always been different. Microsoft doesn't make a free .MPP file viewer available. So if you get a project plan file from, say, your boss or your builder, you might assume that you'll need to acquire a copy of Microsoft Project to open it.
Thankfully, that's not the case. There are plenty of programs and online services, both free and paid-for, that allow you to load and examine a .MPP file.
One of the most powerful is from Housatonic, which you'll find at www.projectviewercentral.com. The company sells various project file viewer solutions, including a system that a company can install on its own internal servers. There's a free demo of that, at the aforementioned web site. Just click on the "Free Online Viewer" menu item at the top of the page and follow the prompts. You can then upload a .MPP file and easily view all aspects of the project.