It's from a parody site, anon.
Granted, it's funny… I think… It's hard to tell of they are trying to make Reagan out to be a retard, or a very sharp man deliberately stating wisdom via a fool's tongue. And it may very well be someone's way of conveying some hidden truths - but it is not a real letter from Reagan.
Or, at the very least, it was first published on a parody news site some time ago and no one has sourced an original paper copy.