Microsoft Project Viewer, Open mpp, mpx

ProjectLibre supports to work with Microsoft Project mpp and mpx files

Here is the supported formats out of the box:

Home:
https://www.projectlibre.com/

Download source for Windows, Linux, Mac:
https://sourceforge.net/projects/projectlibre/files/ProjectLibre/

To successfully run ProjectLibre application you need to install x86 version of Java Runtime Environment (JRE)
https://www.java.com/en/download/manual.jsp

Screenshot:

Microsoft Project examples:
https://www.b4ubuild.com/resources/schedule/b4ubuild_sample_07.mpp
https://www.b4ubuild.com/resources/schedule/b4ubuild_sample_2k-03.mpp
https://www.b4ubuild.com/resources/schedule/b4ubuild_sample.MPX