Where did he hide them? In Jennifer Lopez’s new documentary, The Greatest Love Story Never Told, the “On the Floor” singer — who reunited with ex Ben Affleck in 2021 after breaking off their 2002 engagement — details the duo’s famous romance, which the actor documented in a series of love letters. In one part of the film, Lopez revealed how Affleck had kept all their written communication to one another over the years despite his 2005-2018 marriage to ex Jennifer Garner, whom he shares kids Violet, 18, Seraphina, 15, and Samuel, 11. “This book is a book Ben gave me on our first Christmas back …