Good luck finding one. PDF converts badly enough as it is. It's been discussed many, MANY times on these forums why PDF is a *bad* format to convert from. It should always be a final destination. Especially if it wasn't saved as a tagged PDF.
It takes about 10 minutes to scroll through a 300 page book and press a button to set a style. If you have more books, try to set aside 10 minutes a day (you're probably not reading them back-to-back anyway). Listen to some music or something while you go through it.