For some reason, putting together a simple Illustrator file with a couple mb images produces a pdf of 100+mb. Here’s how to get round it, save the file to pdf with the following options:
1)Uncheck the “Preserve illustrator editing capibilities” box
2)On the Compression tab (left side menu) change the colour bit map image option from “do not downsample” to “average down sampling”, and in the left textbox replace the default 150 with 900, the right side text box will change automatically.
3) Now the compression option below it:-Change it to Jpeg and image quality maximum, and done.
I’ve reduced files from 100 mb to 2mb doing this.
(Solution found at: Illustratorworld.com)