For real CJK fonts, this is often true for monospaced fonts (like MS Gothic, ＭＳ ゴシック ). But many proportional Japanese fonts (like Heisei Mincho ) have variable widths. Specifying Fixed forces the PDF interpreter to ignore the font’s internal widths and use a single width – typically 1000 em units or the /DW (default width) value.
While the string itself is a shorthand or log output, it corresponds to a PDF structure like this: cidfont f1 normal fixed