I thought the same, so I ran brotli and zstd on some PDFs I had laying around.

  brotli 1.0.7 args: -q 11 -w 24
  zstd v1.5.0  args: --ultra -22 --long=31 
                 | Original | zstd    | brotli
  RandomBook.pdf | 15M      | 4.6M    | 4.5M
  Invoice.pdf    | 19.3K    | 16.3K   | 16.1K

I made a table because I wanted to test more files, but almost all PDFs I downloaded/had stored locally were already compressed and I couldn't quickly find a way to decompress them.

Brotli seemed to have a very slight edge over zstd, even on the larger pdf, which I did not expect.

▲ mort96 4 hours ago | parent | next [-]

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    |
    +------+------+-----+------+--------+

▲ Thoreandan an hour ago | parent | prev | next [-]

Does your source .pdf material have FlateDecode'd chunks or did you fully uncompress it?

▲ mrspuratic 3 hours ago | parent | prev | next [-]

> I couldn't quickly find a way to decompress them

    pdftk in.pdf output out.pdf decompress

▲ order-matters 4 hours ago | parent | prev [-]

Whats the assumption we can potentially target as reason for the counter-intuitive result?

that data in pdf files are noisy and zstd should perform better on noisy files?

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jeffbee 4 hours ago | parent [-]

What's counter-intuitive about this outcome?

▲

order-matters 4 hours ago | parent [-]

maybe that was too strongly worded but there was an expectation for zstd to outperform. So the fact it didnt means the result was unexpected. i generally find it helpful to understand why something performs better than expected.

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mort96 3 hours ago | parent [-]

Isn't zstd primarily designed to provide decent compression ratios at amazing speeds? The reason it's exciting is mainly that you can add compression to places where it didn't necessarily make sense before because it's almost free in terms of CPU and memory consumption. I don't think it has ever had a stated goal of beating compression ratio focused algorithms like brotli on compression ratio.

	▲	sgerenser 3 hours ago \| parent [-]
		I actually thought zstd was supposed to be better than Brotli in most cases, but a bit of searching reveals you're right... Brotli, especially at the highest compression levels (10/11), often exceeds zstd at the highest compression levels (20-22). Both are very slow at those levels, although perfectly suitable for "compress once, decompress many" applications which the PDF spec is obviously one of them.