Compressing PDF file can be don on many different ways. This is how I got the best result. By compressing a 14.4MB file with big HiRes pictures to a 2.7MB file for screen use.
In Ubuntu (Linux) open the PDF in a PDF viewer
Print the PDF to a file with the file -> print -> ps, name the file output_print_to_file.ps ( this makes a large postscript file)