PRF (Pixel Run Format, or maybe Pixel Run File) is an obscure bitmap image format associated with the Fastgraph graphics toolkit from Ted Gruber Software. It is also supported by the early-1990s-era IMPROCES software by John Wagner.
The IMPROCES documentation says:
The PRF format is really of use only to programmers who use Fastgraph. I don't recommend using PRF for storing your images if you aren't going to use them in a Fastgraph program. There is no palette information stored with the PRF file.
As if that weren't enough to discourage you from going anywhere near PRF, it also says:
NOTE: Unregistered users of IMPROCES may not use Pixel Run Format files created with IMPROCES in their programs, whether they are registered users of Fastgraph or not.
PRF files apparently begin with (hex) bytes
46 00 41 00 53 00 54 00 47 00 52 00 41 00 46 00.