def compress_pdf(pdf_fpath, output_fname=None):
    """ uses ghostscript to write a pdf """
    import utool as ut
    ut.assertpath(pdf_fpath)
    suffix = '_' + ut.get_datestamp(False) + '_compressed'
    print('pdf_fpath = %r' % (pdf_fpath,))
    output_pdf_fpath = ut.augpath(pdf_fpath, suffix, newfname=output_fname)
    print('output_pdf_fpath = %r' % (output_pdf_fpath,))
    gs_exe = find_ghostscript_exe()
    cmd_list = (
        gs_exe,
        '-sDEVICE=pdfwrite',
        '-dCompatibilityLevel=1.4',
        '-dNOPAUSE',
        '-dQUIET',
        '-dBATCH',
        '-sOutputFile=' + output_pdf_fpath,
        pdf_fpath
    )
    ut.cmd(*cmd_list)
    return output_pdf_fpath
uses ghostscript to write a pdf