Online or the common apps will be printed out?
Yes.
Depends on the AO
File under “what difference does it make.”
Makes a difference. I think they download as a pdf, which is the same as seeing it in hardcopy. I’ve seen messages that XYZ university downloaded the common app. If they download, you can change the common app essay for different groups of universities. I believe you are allowed at least two changes. If they download, you know that the AO is not coming back to look at the new essay (or other details) that you put in for a different group of universities.
Once submitted, you can make changes for the other unsubmitted universities, but those changes are not reflected on any application submitted. Downloading has no impact.
Many schools use Slate to read apps. Slate makes it easy for the AOs to define what info to display, view any uploaded materials, and communicate with HS counselors.