Looks like the PDF is just to show what the messaging interface looks like, and what they've found as a publicly available screenshot is from the crash report involving that plane.

If they logged in, took a screenshot, and published that (even if lots of things are blurred), there's probably more attack surface for some three-letter-agency to bust down their doors and disappear them...

I would guess it limits their ability to be accused of anythign to pick a plane, flight, and time that meets at least three criteria:

1) no passengers on board - you can't be accussed of endangering passengers

2) long past - you can't be accused of anything that happened recently

3) the plan literally no longer exists - you can't be accussed of damaging a plane