brew install ghostscript

#!/bin/bash
[ $# -lt 3 ] && { echo "Usage: $0 /input_dir /output_dir /quality"; exit 1; }
input_dir="$1"; output_dir="$2"; quality="$3"
mkdir -p "$output_dir"
for file in "$input_dir"/*.pdf; do
  base=$(basename "$file" .pdf)
  gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS="$quality" -dNOPAUSE -dQUIET -dBATCH -sOutputFile="$output_dir/${base}_compressed.pdf" "$file"
done

./compress_pdfs.sh /path/to/input /path/to/output /quality_setting