3D Studio Max supports export/save to PNG so it should be able to export its own projects as plain PNG images. Probably used for textures.
Convert Project file to Portable Network Graphic.
We found 3 software records in the database related to .prj to .png file format conversion.
Microsoft Windows software - convert prj to png on Windows
A widely-used 3D animation application from Autodesk
Convert from prj file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned prj to png. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A graphics design tool for making 3D landscaping and animations
Apple macOS / Mac OS X software - convert prj to png on OS X
3D modeling and animation package for Mac.