Library for pdf files editing and combining

Hi,
I need a free or trial version library for editing the pdf files, Here are some specifications for the project:

Problem: Lawers fax in legal documents, these land in my system as
TIFFs and then converted into PDFs. The basic problem is that I
might get three seperate faxes (three seperate files) that comprise a
single Document. Or operators need a way to visually slice and dice
those PDFs into a single accurate file.

CASES:
A) A lawyer breaks up one 500 page document into 5 ,  100 page faxes. We get five files that need to be one file.

B) A lawyer forgets to sign a page, say page #35. We request and get
that single page as another fax, or file in our system. We need to replace page #35.

C) A lawer sends a whole document minus a signature page. They then send that signature page as a seperate file. We need to append it to the document.

You can see there are many cases for which we need human intervention,
so this is to be a VISUAL human intervention tool.

Hope you understood that it is gonna be a pdfs combining software. Need your Help,

Note: library should be workable in  C++ or .NET or VB. You should not send the links unless my requirements are able to be accomplished by that library.