I have a similar document although maybe not as large as the one you speak of. Depending on how you have the document configured (sections, chapters, etc.) you could save portions of the document (in my case, I save them by Chapters) with sequential file names (i.e., Chapter 1, Chapter 2, etc.). You should save the portions from the original file thus retaining the original formatting structure.

In my case, when I finish the document I combine the sequential files into one document and save to .pdf format (for subsequent review/comment from my peers).

That is just how I do it. Others may have different/better solutions.