Your design documentation will probably go through a couple of revisions before it stabilizes. Once it's reasonably stable, and certainly before you go to the next phase of the project, you'll want to place the document (or document set) under change control.
Note that "change control" is not the same as "freezing the specification," which I don't believe is realistic for even the smallest systems. If you plan for the inevitable changes, you'll make your life easier in the long run.
There are several ways to implement change control. I generally resist editing the documents; I've found it much easier to issue change sheets that function as additions to them instead. If the changes are extensive it might be easier to rewrite and replace either the whole document or sections of it, but this is an unusual occurrence.
If it's possible to establish a central location for the master specification, it might make sense to change or annotate the document. You can do this, for example, using the revision tools provided by a word processor such as Microsoft Word. The document could then be placed on a network share or perhaps published on an intranet.
The only problem I've found with putting the document in a central location is ensuring that people are working from the current version of the document and not from hard copies they've printed to scribble on. I confess, I've been guilty of this myself. Often.