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.