Z files have been shar'ed and compressed. To unpack them under unix, first run 'uncompress' on them and then run them through 'sh'. Z files are tarred and compressed. One of the mirror sites converts the files to .gz format. The source code and an msdos executable are available here.
Earlier versions tend to give CRC errors on files.
All of these can be unpacked with StuffIt which it is easiest to obtain from another person.