On the one hand, Microsoft Project files are only editable within this particular software and you need the original application in order to modify them according to your needs. On the other hand, there are view applications available for downloading which provide you with the ability to only view the file. I could recommend MOOS Project Viewer as a temporary solution for viewing the files but in order to edit it you must install Microsoft Project because it not only offers all the control over a file but it has the tools you need in the first place to work on your project files.