Comment by atiedebee

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.