Reducing PDF File size

Open the PDF in Preview, Select File » Save as…, and select the Quartz Filter named Reduce File Size.

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Use ColorSync Utility to fine-tune the filter. Duplicate Reduce File Size and change settings afterwards.

I suggest you first try clearing all values from the Image Sampling block, except Resolution, which should be around 150-300 DPI, depending on how much you want to save.

Inspired from Max Glenister & Milan Kupcevic, Thanks to Burgi, explanation of the example script : It reduce PDF size from Massive to Small using ebook filter

brew install ghostscript # aptitude work too if you do not have brew

compresspdf() {
    echo 'Usage: compresspdf [input file] [output file] [screen|ebook|printer|prepress]'
    gs -sDEVICE=pdfwrite -dNOPAUSE -dQUIET -dBATCH -dPDFSETTINGS=/${3:-"screen"} -dCompatibilityLevel=1.4 -sOutputFile="$2" "$1"
}

compresspdf "Massive.pdf" "Small.pdf" ebook

Gs Options:

-dPDFSETTINGS=/screen   (screen-view-only quality, 72 dpi images)
-dPDFSETTINGS=/ebook    (low quality, 150 dpi images)
-dPDFSETTINGS=/printer  (high quality, 300 dpi images)
-dPDFSETTINGS=/prepress (high quality, color preserving, 300 dpi imgs)
-dPDFSETTINGS=/default  (almost identical to /screen)