Breaking pdf to various images, compress, and recombine
Converting a PDF which is mainly text into images will almost certainly increase the file size, not decrease it. PDFs are quite efficient at containing text — converting to image negates that as you are then just storing images.

Preview includes a Reduce File Size option for PDFs. Open the PDF with Preview, choose File → Export and select Quartz Filter: Reduce File Size. You can also choose Black &amp; White here which may also reduce the file size.

If you really want to see a convert of your PDF to a PDF of images, you can use ImageMagick.

convert /path/to/in.pdf -resize 100% -compress Group4 /path/to/out.pdf