What kind of files will compress well and which ones won ?
In general text files compress best since they repeat the same information over and over allowing the compression algorithm the greatest gains. Fortunately HTML and associated web site files fall into this category, so in general there are good gains to be had. Image files with JPG, GIF, PNG, etc. extensions are already compressed so there is no gain in compressing them again. Also MP3 audio and MPEG video files will not compress well for the same reason.