I've read several times that the modern presidents, post Reagan, have opted to keep their papers private. Does this mean that historians don't get access to them or that they get access only under certain circumstances or only certain documents? Who makes sure only those with proper access get to peruse them?
Why would someone do this?
Generally government documents are kept classified for a period after their creation.
This is for a variety of reason, to prevent secret information being revealed, personal details or so that it does not affect public opinion and hinder governments operations.
In the US the maximum declassification can take is 25 years, though for really secret stuff it may be held longer if it pertains to present activity. So Reagan was the last major president to get all his papers released. We may get the first Bush's papers in a few years.
Generally historians won't be able to look at the papers before their release.