Image Magick is an awesome tool that also does a great work compressing PDF files.
Just run the following command:
$ convert -compress jpeg -density 150 file.pdf file-compressed.pdf
Notice that -compress is the compression type, and -density is the image quality. Also, JPEG compression is lossy, and reduction of DPI usually is lossy. But I managed to have good compression quality with these settings.
There are more details in the Image Magick manual.