pdffiles=$(find /path/to/directory -type -f name *.pdf)
for file in pdffiles; do
  gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dBATCH -dQUIET -sOutputFile=new_$file $file;
  rm $file;
  mv new_$file $file;
done;