Reducing the size of an interactive PDF

I need to reduce the size of a large 25 MG PDF file. It has a degree of interactivity - a bar of tabs at the top that lead to different sections, and within each section there are sub-tabs that lead to different pages. Is there a way to reduce the amount of file space the links take up? Perhaps a way to reduce the number of times they are stored, like a master that only exists once but appears on every page? My client wants the file to be as small as possible - it is 430 pages and about 25 MG.

I have already tried optimizing the file. There are no images in it, so the standard optimization did not change the size at all. I'm fairly certain that anything I can do to reduce the size will have to be structural.

Thanks!