How to downsample images within PDF file?

Need a Java-based solution or, at the worst, command-line for Linux.

I tried to use Ghostscript:

gs -sDEVICE=pdfwrite -dPDFA -dBATCH -dNOPAUSE -dUseCIEColor \
   -sProcessColorModel=DeviceCMYK -sPDFACompatibilityPolicy=1 \
   -sOutputFile=downgraded.pdf leon_range_my12_w22_brochure.pdf

but I got a lot of errors...