The acid content of paper is a good indicator of its service life. It can be measured with a pH value between 0 (acidic, indicating that the paper has a very short lifespan) and 14.0 (alkaline, indicating that the paper has a long lifespan). As the service life increases and the number of exposures to sunlight and air increases, the acidity of any kind of paper will also increase. The acidic pH value is below 7.0. For example, the newspaper you read every day is made of acid paper. If you can preserve it carefully, then this kind of newspaper can probably reach a lifespan of 20 years, but if you expose the newspaper to sunlight and air, then It may only be kept for a few weeks.
Acid-free means that the pH of the paper is 7.0 (neutral) or higher (alkaline). Under normal use and storage conditions, the service life of acid-free paper can reach about 200 years.