A question we got a lot from our compression customers is "shouldn't I also compress PDFs and image files?" A very logical question, seeing as they are generally the bulkiest part of your site's Web transfer and bandwdith costs. While httpZip and ZipEnable can compress any file type you wish, they do not compress PDFs and images by default for a very important reason -- most such files are already compressed when they are created, and thus re-compression can introduce distortion or even make the files bigger!
So, we were excited to see Andy King's excellent guide to optimizing PDFs for Web delivery:
http://www.websiteoptimization.com/speed/tweak/pdf/
He offers a solid set of guidelines for optimizing PDFs, converting fonts, forms and colors to their most compact forms, and how to avoid "refried graphics." And better yet, he's got a handy version-by-version guide to Adobe Acrobat functionalities.
So, if you've got a site with bulky files that aren't good candidates for HTTP compression, you might discover other good ways to optimize them for delivery.
If you're wondering "who's Andy King?" his site is an absolute goldmine for anyone concerned with site and server performance -- be sure to check it out:
http://www.websiteoptimization.com