Hi Mark,

Im quoting this thread on stackoverflow with the exact same question.

The flag that works best for me in reducing file size is 

--image-dpi 300

As the default DPI is 600, this can reduce the filesize of an image-heavy PDF by 75% and is still good enough for most purposes.

There are two other flags you can use:

--lowquality - supposedly makes it smaller

--image-quality 80 - jpeg compression, default is 94

but overall I find the DPI makes the biggest difference, and if you aren't printing to 600dpi or zooming then you're not really losing anything.

For your use case also try

--no-outline - as you likely don't need a contents section

and the self-explanatory

--disable-internal-links

--disable-external-links

but these are likely to be smaller wins.

Hope this helps

Guilherme