Default files are searched in the data directory, which can be set via `--data-dir`. However, this option affects the `-d`/`--defaults` options that come *after* `--data-dir`.
Example:

pandoc -d pdf --data-dir=./data

will look for the file `defaults/pdf.yaml` in the default data directory, but

pandoc --data-dir=./data -d pdf

will check for `./data/defaults/pdf.yaml`.