The info I'm finding on the net is that this indicates the font you're using is a Postscript Type 1 font, which XeTeX doesn't fully support. It should work if you use is a TTF or an OTF font. (The file name should have a `.ttf` or `.otf` file extension. Type 1 fonts will have `.pfb`, `.pfm` or `.afm`, according to Wikipedia.)
Thanks for clarifying. However, the file name for Courier/Courier Standard on my Windows 10 is actually "coure.fon" (it was tricky to find out, because that particular dir behaves very differently from all others), but the same principle holds: only "modern" font formats seem to work.
There sure are a lot of death traps on the way to converting a PDF document! And this is for a fixed-letter-size, fixed-column-width, plaintext document... After the nightmare I went through a few years ago, I won't ever even *try* to convert a fancy HTML + CSS document to PDF ever again!