when times go on everything changes, in the same way some chemical reactions occur in paper and it turns to yellow as when times go on food items taste get change.
Paper is made from wood, which is largely white cellulose. Wood also has a dark substance called lignin, which ends up in the paper. The exposure of lignin to air and sunlight is what turns paper yellow. Lignin eventually turns paper yellow because of oxidation i.e. the lignin molecules, when exposed to oxygen begin to change and become less stable.
I accept the points stated by sarala and i think it has a valid point and as a result of paper being prepared from wood and even the wood also turns from green to brown i mean the stem or bark of a plant.
