Ran the tests again with some more files, this time decompressing the pdf in advance. I picked some widely available PDFs to make the experiment reproducable.

  file            | raw         | zstd       (%)      | brotli     (%)     |
  gawk.pdf        | 8.068.092   | 1.437.529  (17.8%)  | 1.376.106  (17.1%) |
  shannon.pdf     | 335.009     | 68.739     (20.5%)  | 65.978     (19.6%) |
  attention.pdf   | 24.742.418  | 367.367    (1.4%)   | 362.578    (1.4%)  |
  learnopengl.pdf | 253.041.425 | 37.756.229 (14.9%)  | 35.223.532 (13.9%) |

For learnopengl.pdf I also tested the decompression performance, since it is such a large file, and got the following (less surprising) results using 'perf stat -r 5':

  zstd:   0.4532 +- 0.0216 seconds time elapsed  ( +-  4.77% )
  brotli: 0.7641 +- 0.0242 seconds time elapsed  ( +-  3.17% )

The conclusion seems to be consistent with what brotli's authors have said: brotli achieves slightly better compression, at the cost of a little over half the decompression speed.