I am currently writing a paper which is partly based on the unpublished work of a known mathematician. The "unpublished work" is basically a memo that has not appeared in any journal, but has widely circulated since its release and is available on the "Publications" list of the author's homepage.

I would really like to use this article in my paper, but I am usually against referencing an unpublished "memo", for obvious reasons. Since this is a different case (a high-quality paper from a known mathematician), how do I proceed with including it in my paper?