reduce pdf file size

I do not know of actual programs for compressing .pdf files as .pdf however your best bet would simply be to try using alternative PDF creation utilities as I have seen file size change dramatically between them.

I have never done a 8MB PDF, however I did do a 2MB one and I noticed that by using Bullzip, Flashpaper and the official Adobe Acrobat created a diffrence of over 60% in some cases... Generally, Adobe came in best, Flashpaper second and Bullzip last. (thre are a load more tools available, but these are the ones I tested.

Some of these utilities also enable you to compress the resources and control the quality. Whilst Bullzip is free, you may want to try a demo of the others and see if they can help you.

We can add some opensource program for dealing with images as gimp or imagemagick.

The first, GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. Something like Photoshop but opensource. Huge program: you can use e.g. to create a pdf from scanned images.

The second, Imagemagick is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats (over 100) including DPX, EXR, GIF, JPEG, JPEG-2000, PDF, PNG, Postscript, SVG, and TIFF.

With the second it's possible to convert in pdf using expressly the option -compress jpeg specifying the geometry and size:

convert -geometry 2480x3508 -compress jpeg  Orig.pdf  -page 2480x3508  Final.pdf

Note that you may want to decide with which definition to have the final document: in the above case I choose A4 format 300 dpi. Below a table to remember those dimensions in pixel for each definition of oint Per Inches, PPI.

Paper   PPI  Width  Height
---------------------------
A4      72    595    842
A4      200   1654   2339
A4      300   2480   3508
A4      400   3307   4677
A4      600   4961   7016
A4      1200  9921   14031
---------------------------
Letter  72    612    792
Letter  200   1700   2200
Letter  300   2550   3300
Letter  400   3400   4400
Letter  600   5100   6600
Letter  1200  10200  13200

Convert is a command that comes with Imagemagick.