There are two nice utilities to reduce the file size of PDF documents in Ubuntu:

Download the following zip and extract the .sh file

shrinkpdf.sh

Then issue the following command:

./shrinkpdf.sh in.pdf > out.pdf

Or you can issue the following command:

gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS="/screen" -sOutputFile=output.pdf input.pdf