The easiest way to shrink the file size of a PDF in Acrobat 8 is to click on Document > Reduce File Size. For the compatibly setting I would recommend Acrobat 6 and later. The higher compatibility setting you choose, the smaller the file size will be. The downside of choosing a high compatibility setting is that people with older versions of Acrobat will not be able to open the document. For that reason, Acrobat 6 and later is usually a safe bet.