Do you have acrobat pro? And what is the purpose of reducing file size. Is it just for emailing purposes? You could either use adobe send now to keep the quality as it was, my best method for reducing the file size is to change the color space to sRGB as that've keeps the quality the same but massively reduces the file size.
or alternatively use PDF optimiser for fine tuning, file - save as - optimised PDF
hope this helps?