I personally prefer ThinkFree. Battery drains quickly if the screen is on for a long time, so the battery should drain quickly if you are using any app to read pdf. Try reducing screen brightness. You can try ebook readers like Aldiko (which is widely considered the best eBook reader), but you need to update it to read pdf files. Anyway, I really like the reading view offered by Thinkfree Office. In my personal opinion, Thinkfree is the best one out there.