I am on the waitlist for Bing and can't check directly--would it also answer specific questions about the doc? (Rather than summarize.)

Our site is meant for Q&A, and has a layer of tech that finds the sections in the large document that are relevant to the Q first. This will not work well in general for summarization on unstructured content. But most content tends to be structured and in practice we are finding that the approach still works on e.g. news articles, wikipedia articles, blog posts. It's almost as if where it doesn't work, a human would have trouble too. (e.g., on a long rambling HN thread).