Outline PostScript printer font (ChiWriter)
PSF is a file extension for an Outline PostScript printer font file format used by ChiWriter. ChiWriter is a DOS-based multi-font scientific word processor that was developed and distributed commercially till 1995.
ChiWriter was one of the first WYSIWYG editors that could write mathematical formulas on the PC.
The primary association for the PSF file extension is a Photoshop preference storage format file.
Learn more about PSF files:
Wikipedia has an entry on ChiWriter.
Browse the printing FAQ at IT Knowledge Exchange.