A document that large must have some kind of a structure that you and your team can use to break it apart into smaller, more manageable chunks. One thing to consider is to place the content into a content management system of kind (a wiki comes to mind) to allow the content to more easily be maintained and grow without the limitations Word creates.

The decision may not have been up to you to keep it all in one Word document but making the case to either break apart the document or find a better system in which to maintain it ought to be easy in the face of the usability issues you mention.