Quick question: What is the caching / download mechanism like in bookreader? Does it only download each page as requested, cache a few pages around the current

Quick question: What is the caching / download mechanism like in bookreader? Does it only download each page as requested, cache a few pages around the current page, or download the whole thing before display? I am looking for something better than what we have cooked up to avoid downloading an entire pdf every time somebody wants to see a couple pages of it.