Are you sure? Admittedly I only have 1 PDF in my homedir, but no combination of flags to zstd gets it to match the size of brotli's output on that particular file. Even zstd --long --ultra -22.

on max compression (11 vs zstd's 22) of text brotli will be around 3-4% denser... and a lot slower. Decompression wise zstd is over 2x faster.

The pdfs you have are already compressed with deflate (zip).