st/exp is a file compression utility for DOS, and associated .PRS file format. st is the compressor, and exp is the decompressor. It's designed for good compression of text files.
It was developed by MicroComputer Square. It is part of a software collection named FlySpeed. FlySpeed seems to be something like a brand name.
The original filename is not stored in the compressed file. Decompressed files are given the filename extension .exp.
Unknown. PRS files have no obvious signature.