The original "question" reminds me of Grouch Marx's question to Chico Marx in one of their movies: Why a duck?

The answer is quite simple once you have more information to form an opinion.

For you question we might want to know things such as: How were you getting Sigil to "download" (ill-used word) the HTML file converted from Word? Isn't there a 300 kilobyte limit on the size of a chapter in an epub file (I suspect a 328 page Word document in HTML would exceed that)?