Asking a celebrity Prof to sign a paper they wrote?
Just ask! In the past, before electronic publishing, one had to ask for physical reprints from authors. They usually signed them with a nice greeting, sometimes a signature, and occasionally nothing. I have sent reprints to other upon request and always with a signed greeting because I appreciate the request. I have a hard time seeing anyone being offended by it although an eyebrow may be raised by the request since it is now quite unusual. But it is a good way to strike up a conversation and contact the person. Personally I would try to approach the person away from an "audience" in order to not interrupt and have a real chance to state why you admire the work (person). I am sure some might say no but I would not be offended if someone asked me, not that I see it happening to me any-time soon.