Paper was normally made out of linen, rags, cotton, or other plant fibers. Asian-style papers are made from the core of tall grasses (bast fibers) and Western-style papers are made from 100% undyed cotton and linen rags or cast-off fibers from spinning. After being picked or washed clean of impurities, the long fibers that are left are pure cellulose, which is actually colorless, but reflects light opaquely and we see the color white. It wouldn't be until the mid-19th century when paper was made out of wood fiber.

In 1844-45, two individuals invented the wood paper-making process. A Canadian, Charles Fenerty, and a German, Friedrich Gottlob Keller, both involved in lumber industries and recognized the cost and durability that wood pulp provided over cotton. Within thirty years, wood pulp paper was all the rage on both sides of the pond. While wood pulp paper was cheaper and just as durable as cotton or other linen papers, there were drawbacks. Most significantly, wood pulp paper is much more prone to being effected by oxygen and sunlight.

Wood is primarily made up of two polymer substances – cellulose and lignin. Cellulose is the most abundant organic material in nature. It is also technically colorless and reflects light extremely well rather than absorbs it (which makes it opaque); therefore humans see cellulose as white. However, cellulose is also somewhat susceptible to oxidation, although not nearly as much as lignin. Oxidation causes a loss of electron(s) and weakens the material. In the case of cellulose, this can result in some light being absorbed, making the material (in this case, wood pulp) appear duller and less white (some describe it as "warmer"), but this isn't what causes the bulk of the yellowing in aged paper.

Lignin is the other prominent substance found in paper, newspaper in particular. Lignin is a compound found in wood that actually makes the wood stronger and harder. Lignin is a dark color naturally (think brown-paper bags or brown cardboard boxes, where much of the lignin is left in for added strength, while also resulting in the bags/boxes being cheaper due to less processing needed in their creation). Lignin is also highly susceptible to oxidation. Exposure to oxygen (especially when combined with sunlight) alters the molecular structure of lignin, causing a change in how the compound absorbs and reflects light, resulting in the substance containing oxidized lignin turning a yellow-brown color in the human visual spectrum.

Since the paper used in newspapers tends to be made with a less intensive and more cost-efficient process (since a lot of the wood pulp paper is needed), there tends to be significantly more lignin in newspapers than in, say, paper made for books, where a bleaching process is used to remove much of the lignin. The net result is that, as newspapers get older and are exposed to more oxygen, they turn a yellowish-brown color relatively quickly.

As for books, since the paper used tends to be higher grade (among other things, meaning more lignin is removed along with a much more intensive bleaching process), the discolorization doesn't happen as quickly. However, the chemicals used in the bleaching process to make white paper can result in the cellulose being more susceptible to oxidation than it would otherwise be, contributing slightly to the discolorization of the pages in the long run.

Today, to combat this, many important documents are now written on acid-free paper with a limited amount of lignin, to prevent it from deteriorating as quickly.

http://gizmodo.com/why-old-paper-turns-yellow-1692099465
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