It might be useful to post a zip of your current document, so we can see your setup to advise best based on what your nav and content show.
My guess though is that you probably should look into "fixed" or "sticky" navigation. It just requires a little bit of CSS (non-JS) code to make a nav bar that can stay in place while the rest of the page is treated the same. There's also some JS code that can make it work a bit better.
Here are a few posts with some solutions, though you can probably find more searching through the forums: