1. Open it in a free online viewer
The quickest way: drop the file into the MPP Viewer. It shows the Gantt chart, every task with its WBS number, durations, start and finish dates, percent complete, predecessors and the people assigned, the way Project lays them out. Summary tasks collapse and expand like in Project.
Viewing is free and needs no account. If you need the plan in a spreadsheet or as a PDF to forward, the export to Excel, CSV and PDF is $19 once.
2. Ask the sender for a PDF or Excel copy
If you only need to read it once, the sender can use File, Export in Project to save a PDF or an Excel workbook. This works, but you get a snapshot: every time the plan changes, you have to ask again.
3. Get a Project license from work
If your company already pays for Microsoft Project, ask IT for a seat: Project Plan 3 and Plan 5 include the Windows desktop app, which opens and edits .mpp files. It's the right answer if you'll build plans yourself, and a lot of cost if you only need to read them.
4. Install a desktop app
Desktop apps such as ProjectLibre (free, Java based) and several paid MPP viewers read .mpp files on Windows and Mac. They make sense if you open plans every day and want to edit them. For reading the odd plan someone sends you, a browser viewer is less to install and nothing to update.
What you can and can't see without Project
- You can see: the task list and its outline (WBS), dates, durations, progress, milestones, links between tasks, resources, baseline dates and task notes.
- You can't: edit the plan and save it back as .mpp. For that you need Project or a planning tool that writes .mpp. A viewer is for reading, checking and sharing.
- Password-protected files can't be opened by anything except Project with the password. Ask the sender for a copy without one.
Other Project file types
.mpx is the old exchange format from Project 98 and earlier. .xml is the Project XML format that Project and many other tools save with "Save As, XML". The MPP Viewer opens all three.