Microsoft Project is Project Management Software.
Project files starting with Project version 4, are in the "OLE2" document container format. Project files do not appear to have moved to the later XML format like other Microsoft Formats and remain as OLE containers.
The format appears to remain the same since Project 2010, and newer saved formats can be opened in the older versions.
An MPX or Microsoft Project Exchange file format (ASCII) can be exported for exchanging data with other PM software.
Versions & Compatibility
|95 (4.1) Project
DOS vs Windows
Microsoft Project DOS files are very different from Windows files. Below is a from Microsoft Project 1.0 for Windows help:
The conversion utility that comes with Microsoft Project for Windows converts files from Microsoft Project for MS-DOS version 3.0 or 4.0 into files that can be loaded directly into Microsoft Project for Windows. Because the functionality of the two products is not the same, some information may be lost or changed in the conversion process.