Check in via Lufthansa app

Since I'd like to choose seats and avoid overbooking I'd like to check in in advance.

There is an app and they say that we can check in using it.

My questions are:

1. if I use app to check in should I print something else to show at the airport?

2. what to do when I arrive at the airport? Should I use my mobile to scan some code?

Edit: Since I'll not have access to a printer to print my boarding pass after checking in online (if I did), I'm thinking to use app. But I have to report my company about my flight so I need boarding pass.

3. Is it possible to export as pdf the boarding pass from the app and print it later?

Edit 2: from here they say:

Should you have any problems with your mobile boarding pass you can, of course, collect a conventional boarding pass from a check-in machine or from a Lufthansa check-in counter at the airport. The same applies if, for example, your mobile phone’s battery is flat or you have inadvertently deleted your mobile boarding pass.

4. Do you understand that we can check in using app and also print the boarding pass at the airport machines (a kind of second copy)?