Thanks to all for the comments. The thing is an OBJ wrapped up as a PCF--apparently there were something like Poser Container Files before my time with poser. Someone on Windows is checking to see if she can convert it for me, so my fingers are crossed.

It's not for something I need. I just came across what looks like a nice low rez '30s biplane pilot that was offered for free, and now only exists on the internet archive. You can see it here if you're curious: https://web.archive.org/web/20190707030807/http://thierry.perrain.free.fr/Downloads.html

Curiously, the biplane that's available for him just has an OBJ file, so it opens fine.