want some advice about android/iPhone app

Hi everyone, I’m new here.
I heard that there is a lot of promotion activities recently.
Because of my work, I have to use phone in most of the time, so a useful software is important to me.
As a man who is not in favor of shopping, I prefer to change some software in my phone which are mostly outdated.
Does anyone have any useful tool to recommend? Such like something that can manage document or contacts easily.
Thanks a lot.:)