Terminal/GroupFileYearMonthByDateModified

cd ~/Downloads/"PDF FOLDERS" && find . -maxdepth 1 -type f -not -name ".DS_Store" -print0 | while IFS= read -r -d '' f; do y=$(stat -f "%Sm" -t "%Y" "$f"); m=$(stat -f "%Sm" -t "%m" "$f"); mkdir -p "$y/$m"; mv -- "$f" "$y/$m"/; done