Everyone knows it means: everyone knows it.

The certificate is a special personal item, representing a person's special life and work experience. For example, marriage certificate, divorce certificate, military officer certificate, and college graduation certificate. Personal hard work and ups and downs in love and marriage should be unknown to others.

So the original text should be: Although these documents are inconspicuous, there are little-known stories hidden behind them.

Little known and well known mean exactly the opposite.