mac下安装 brew install ghostscript
    :param pdf_in:
    :param pdf_output:
    :return:

def compress(pdf_in, pdf_output):
    """
    mac下安装 brew install ghostscript
    :param pdf_in:
    :param pdf_output:
    :return:
    """
    cmd = 'gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -sOutputFile=%s %s'
    cmd = cmd % (pdf_output, pdf_in)
    os.system(cmd)