Introduction
The compression is only interesting for a small range of file sizes. That is, compressing a 3 MB file to two diskettes remains a viable project, but takes too much time if you have to do it everyday or several times in a day. You have in this case to use other transfer tools, like Zip cartridges or even CD-ROMs. The throughput is much better, as you don't need the compression any more.

Self-Extracting Archives
When you don't know if the person to which you're sending an archive has or not some decompression program, the solution is often to transform the archive and make it a self-extracting archive (SEA archive on the Mac, exe file on the PC). This is not a good solution when you exchange data across platforms. The decompression engine cannot run on the other platform!!
You have in this case to find the program that could uncompress the first archive. Some programs know how to ignore the decompression engine and access the raw data.
See also the pages we publish on MacBinary files and SEA archives.

Standard Compression Format on the PC
The standard compression utility on the PC is PKZip/WinZip. Its files have a '.ZIP' extension. On the Macintosh, you will have to use ZipIt! or Stuffit Deluxe to expand these files. The registered version of ZipIt can even decompress multivolume archives (that is, a compressed file spanning several floppy disks).
However, it appears that you can't just put the floppy disks in the drive and launch ZipIt on them. ZipIt insists on creating the target file in the same folder. As all the compressed files on the floppy disks have the same name, you can't just copy them in a subfolder. The solution is to copy the first file on the hard disk in the target folder, to launch ZipIt on this first file, and to redirect the program to the others when it asks for it.

Standard Compression Format on the Mac
Conversely, the standard compression utilities on the Macintosh are Stuffit and Compactor. There are several freeware and shareware programs that can decompress these files (Unstuff and Unstuffit come to mind for the first format. I have still to find something for the second). See Allume Systems Web Site (formely known as Aladdin Systems). Please note that the standard edition of UnStuffit (not the free one) may be asked to produce a MacBinary file instead of merely decompressing the data fork. Such MacBinary files can then be exploited by MacDisk or MacImage to produce Macintosh applications on magnetical media or on CD-ROM.

Other Compression Schemes
I have also seen files that are compressed on the fly by Disk Doubler, a resident program (using the signature DDAPDDFL, the file begins with the magic signature 0xabcd). I don't know any program which could uncompress those files. The best thing is to ask the sender to resave the file uncompressed.