How can I improve text on a yellowed/blotched page?

[j4nd3r53n]

I am working on scanning some historical texts, in Chinese - unfortunately the pages are somewhat yellowed and blotched, like the one attached. Is there a easy way to make the background paler - or even completely white - while preserving or darkening the ? I'm preparing the photo for OCR; so far I have only managed to digitalize a minor part of the text.