[RPG] legally obtain AD&D 2nd core rulebook in a digital form

I think the two things you have to do "manually" are monsters and treasure conversion, which will actually probably be more of a "re-imagining" than a conversion. That can be fairly labor intensive, though, and I don't know a way around it. Using the compendium and monster builder can definitely help in tracking down and/or creating equivalent monsters, though. I recently used the monster builder to rebuild myconids from scratch to make them like 1e myconids, and it actually went pretty smoothly.

Although you said you don't want to buy new source books, there is a Ravenloft book coming out for 4e that puts the setting in the Shadowfell.

Not as of October 2017.

Per the Mike Mearls interview of 3 November 2014:

We're definitely looking at PDFs, ebooks, and other digital platforms, but no news yet. The goal with anything along those lines will be grow D&D, not just sell ebooks to people who already play the game, so we're putting a lot of work into figuring out that side of the equation.

It is likely that if and when a PDF version is made available, it will appear on http://www.dndclassics.com/ alongside the other PDF material from Wizards of the Coast.

January 2016 Update: The 5E system reference document is available in PDF format. For many people this may fulfill the need for reference material (looking up spells and creatures, etc)

October 2017 Update: While a PDF version is still not available, there are now a number of options for electronic reference.

• The previously mentioned SRD.
• D&D Beyond provides the official material in compendium format.
• Fantasy Grounds has officially licensed material that covers the core rulebooks.
• Roll20 also has officially licensed material though at the time of this update still lacks some of the core rule content.