Compression is all dependent on what you're going to do witht he final document. Professional printing will require a bigger file, with less compression, than a pdf that's is just to be viewed on a monitor.
One quick trick with any pdf is to go to File menu/Reduce file size. That will remove a lot of redundant stuff and usually give you a much smaller file.
For creating new pdfs, go to Advanced menu/Acrobat distiller. You can just choose any of the Settings from the drop down menu. Screen will produce small files. Print will produce bigger files for printing on a desktop printer. Press is higher quality for pro printing. The X types are for pro printing only and you probably don't have to worry about those. Whcihever setting you choose will stay as the default for subsequent pdfs until you change it.
Using OSX 10.3.9 on a G4