This can reduce files to ~15% of their size (2.3M to 345K, in one case) with no obvious degradation of quality.
ghostscript -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/printer -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
Other options for PDFSETTINGS:
- /screen selects low-resolution output similar to the Acrobat Distiller "Screen Optimized" setting.
 - /ebook selects medium-resolution output similar to the Acrobat Distiller "eBook" setting.
 - /printer selects output similar to the Acrobat Distiller "Print Optimized" setting.
 - /prepress selects output similar to Acrobat Distiller "Prepress Optimized" setting.