I have to write a 90 page document on a novel drug synthesis that I am undertaking in my year in industry. Time constraints aside is this style of working comparable to a PhD? Will this set me in good stead for PhD'ing?

It's not possible to comment on whether this is like a PhD (you'd need to give far more details than this forum has room for). However learning to write scientific reports will stand you in very good stead, there's an art to scientific writing, but once you've mastered it you're laughing.