This can reduce files to ~15% of their size (2.3M to 345K, in one case) with no obvious degradation of quality.
ghostscript -r72 -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/default -dNOPAUSE -dQUIET -dBATCH -dCompressFonts=true -sOutputFile=output.pdf input.pdf
Change -r for the resolution.
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.