Compress existing PDF using C# programming using freeware libraries

Question

I have been searching a lot on Google about how to compress existing pdf (size). My problem is

1. I can\'t use any application, because it needs to b

Answer 1

GhostScript is AGPL licensed software that can compress PDFs. There is also an AGPL licensed C# wrapper for it on github here.

You could use the GhostscriptProcessor class from that wrapper to pass custom commands to GhostScript, like the ones found in this AskUbuntu answer describing PDF compression.