Ok, saved a lot of explanation.
DC is sometimes "everything since 2015" and sometimes it means just the continuous version. (Subscription for the paid software). You will see it used in either way.
To understand 2015 and 2017 look at the commercial software. 2015 was a paid for release of Acrobat (first called "12"!) It had updates but no new features. Later there was 2017 to catch up with some of the new features added for subscribers. Although Reader is free, it has essentially the same release structure as the paid software.
So... for Reader it 2017 was a bunch of new features. It continues with bug fix updates. This would be your starting position today: only someone who chose to freeze on an old version would use 2015. Most full releases are only EXE files. The Enterprise Deployment Guide talks about getting an MSI from that. You can expect to need to use an MSP to the latest version, and of course the whole point of the Classic track is that you choose when to install updates i.e. MSPs. Since they have critical security updates you'll want a deployment system for them.
Whew!