We all know that most papers are made out of wood and that wood contains two substance in abundance, cellulose and Lignin.
With time, as these two substances are exposed more to oxygen and sunlight, they go through the natural process of oxidation which results in loss of electrons.
The more electrons that the paper loses, the more yellow – brown it starts to appear.
That is why over the decades, important documents have been written or printed on acid-free papers which have less amount of Lignin thus making it last longer.
Here is a video explaining the whole thing in detail.