How do I put an app in the background instead of closing?

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http://forum.xda-developers.com – Hi, I am very annoyed with Google drive combined with pdf readers! I put my reading materials (pdf) on my Google drive account (offline mode) but every time when I switch focus to something else like music player PDF reader closes and then I have to load from Google drive again and scroll back to my last Page. I don't want my pdf reader to close but run in the background like connectbot! Backgrounder on i phone works great. My nexus 7 isn't rooted btw. Sent from my Nexus 7 using xda app-developers app (HowTos)