I read in a paper a while back of a lady who preserved news paper clippings by brushing them with milk of magnesia. the article said that it took the yellow out and kept the clippings in good shape, the clippings were 30 years old. Newspaper is full of acid and yellows quickly. Perhaps neutralizing your brown spots would stop them from spreading.
The proper thing to do would be to contact a restorer or research articles on preservation on the internet.
But if one were adventurous, I wonder if one brushed a little bit on the back of one the stains from the back of the print and gently buffed it back off whether it would help. I’d try it in an inconspicuous spot first and experiment with the length of time the milk of magnesia stays on.
What is so terrible about art is that the further one advances in it, the more tightly one is committed to an extreme and nearly impossible objective.
Rilke