Does software enter public domain?

According to current US law, no software created since the popularization of the personal computer will become public domain before 2050. And very little of it before 2070. See
http://www.copyright.gov/help/faq/faq-duration.html#duration for details.

What would happen to the things once software like adobe's start going into public domain?

How does that work? Is something like Adobe's 1980 pdf viewer required to have the source code released at whatever point it enters?