YARMOUTH, Mass.  — Ryan Cooper was a 20-something Californian unsure of his place in the world when he struck up a pen pal correspondence in the 1970s with Otto Frank, the father of the young Holocaust victim Anne Frank.

Through dozens of letters and several face-to-face meetings, the two forged a friendship that lasted until Frank died in 1980 at the age of 91.

Now 73 years old, Cooper, an antiques dealer and artist in Massachusetts, has donated a trove of letters and mementos he received from Frank to the U.S. Holocaust Memorial Museum in Washington ahead of the 90th anniversary Wednesday of Anne Frank’s birth on June 12, 1929.

He wants the letters to be shared so that people can have a deeper understanding of the man who introduced the world to Anne Frank, whose famous World War II diary is considered one of the most important works of the 20th century.

“He was a lot like Anne in that he was an optimist,” Cooper said of Otto Frank at his house on Cape Cod recently. “He always believed the world would be right in the end, and he based that hope on the young people.”

As the German army occupied the Netherlands, the Franks hid in the attic of Otto Frank’s office in Amsterdam. But they were eventually discovered and sent to concentration camps, where 15-year-old Anne, her elder sister and her mother died — among an estimated 6 million Jews killed by the Nazis.