As Jean-guy said, you really do need to be more explicit. The exact
wording of the error message you receive would be helpful.
File size rather than page count is how the bigness of a document is
described. Do the elements in it come from many sources, or were they
all created at home? in Word, other Office components, or from other
apps?
But if splitting it into smaller documents is a workable option, that
would probably go a long way toward avoiding future problems. (Do NOT
try to use Master Documents. MS no longer even bothers to tell you
how, but they've never tried to fix it, either.)