$ sudo lsof | grep '/Documents/'

$ sudo lsof | grep '/Documents/'
DiskSpace  6071        joeuser    3r      REG                1,5     106496 11534741 /Users/joeuser/Library/Containers/com.apple.iBooksX/Data/Documents/BKLibrary/BKLibrary-1-091020131601.sqlite
DiskSpace  6071        joeuser    4r      REG                1,5     762232 11534745 /Users/joeuser/Library/Containers/com.apple.iBooksX/Data/Documents/BKLibrary/BKLibrary-1-091020131601.sqlite-wal
DiskSpace  6071        joeuser    5r      REG                1,5      32768 11534746 /Users/joeuser/Library/Containers/com.apple.iBooksX/Data/Documents/BKLibrary/BKLibrary-1-091020131601.sqlite-shm

$ pgrep -fl DiskSpace
6071 /Applications/Books.app/Contents/PlugIns/DiskSpaceEfficiency.appex/Contents/MacOS/DiskSpaceEfficiency