Why is it important for paper or board to be acid-free and lignin-free?

An acid-free and lignin-free paper or board is a purified material that will not break down over time and become acidic. A paper or board that is only acid-free and still has lignin content will become acidic. Lignin is an organic substance found in wood pulp. The lignin breaks down into acid and will yellow or discolor the paper or board and whatever else contacts the paper or board.