Well your question is interesting and there are multiple aspects to PDF compression. Data in a PDF file can be compressed in many ways. First there is compression for the images (so JBIG, JPEG, LZW, Flate etc). Then there is compression of the actual PDF stream (simply refer to the PDF reference and it does a good job explaining it). Finally there is optimizing PDF compression, where in addition to pdf data compression, prior to actually performing the compression to the PDF data stream you optimize the data to yield better pdf data compression. What is âoptimizingâ the PDF data for better compression? Well it can vary depending on the PDF creation software. An example is yielding better compression for images. Instead of simply taking an image and arbitrarily compressing it using JBIG, JPEG etc compression you optimize the image (for example slicing it up into smaller pieces) so that it yields better compression when applying any of the standard image compression types (again JBIG, JPEG etc). So there are multiple aspects to PDF compression technology which may not be obvious. The down-side with taking an image and slicing it up means, you have no clue what the original image was (if you try to recover the image). Hope this helps. This is just a simple introduction - you need to refer to the PDF Reference for the insane details.
As for the actual files and encoding. The main thing is that the first (and only) version was named CompuServe Compress 4.0. A lot of the other encoding are slightly different. (The compression was first done on a “compress-4.0.10.zip” file, later the name changed to compress-0.2.9. I don't have that version but if you have it you can extract all files to be used with compress.skew for testing.) I'd suggest creating an image viewer and looking at the compression that you find. If there are major errors than those will have the highest compression levels, if there's not they will all have a much less efficient compression. Don't worry about making sure compression is right. The easiest place to look are the data tables (see below), and the data table for your image viewer. If you find that there is a major error in the tables then it will take a lot longer To fix it. Sometimes you'll find.