EDIT: Something weird is going on here. When compressing zstd in parallel it produces the garbage results seen here, but when compressing on a single core, it produces result competitive with Brotli (37M). See: https://news.ycombinator.com/item?id=46723158

I did my own testing where Brotli also ended up better than ZSTD: https://news.ycombinator.com/item?id=46722044

Results by compression type across 55 PDFs:

    +------+------+-----+------+--------+
    | none | zstd | xz  | gzip | brotli |
    +------|------|-----|------|--------|
    | 47M  | 45M  | 39M | 38M  | 37M    |
    +------+------+-----+------+--------+