Compress existing PDF using C programming using freeware libraries

Question

I've been looking for ways to compress existing pdf files on Google (size). My issue is this:

1. I'm unable to use any application because the task requires a C# programme.
2. My clients don't want to go over budget, so I can't use any paid libraries. 

I worked on my homework for the past two days and came up with a solution utilising iTextSharp and BitMiracle, but it was in vain because the former only reduces 1% of a file and the latter is a premium programme.

I also came across PDFcompressNET and pdftk, however their.dll files were nowhere to be found.

The file is actually an insurance policy with 2-3 images (black and white) and roughly 70 pages, totaling 5 MB.

I just need the output in pdf format (no other formats are OK).