Reducing the size of a black-and-white scanned pdf
A great PDF compressor is Ghostscript (gs), which is native in most Linux distributions and easily installable for Mac OS X and Windows. 

Daniel summarizes summarizes it best here, with the key quote being his preferred options: 

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET -dBATCH \
    -sOutputFile=foo-compressed.pdf foo.pdf