BERN, Switzerland—Albert Hofmann realized he had invented LSD after a vivid experiment in 1943. The Swiss chemist retired a few decades later, and his personal archive began a long, strange trip that ended at a quiet institute in this leafy city—where it is looked after by a part-time dairy farmer.

Beat Bäche, who is writing a book about hallucinogen-producing fungus when he isn’t milking cows at a farm where he works, curates Dr. Hofmann’s papers. That is because Mr. Bäche is nearly the only person to use the archive since it arrived at Bern’s Institute of Medical History in 2013.

Just one other scholar, a student from Zurich, showed up briefly last year.

Although Mr. Bäche doesn’t officially work there, the institute directed questions about the archive to him. On a recent day, he riffled through items including Dr. Hofmann’s formulas and photos, his correspondence with psychedelics advocate Timothy Leary, and a presentation for the Swiss army on military uses for the drug.

Roger Liggenstorfer, a friend of Dr. Hofmann’s, says the late chemist wanted researchers flocking to his archive. The current situation is “not really the wish of Albert,” he says.