docs/bookdown_example/_output.yml

bookdown::gitbook:
  table_css: false
  config:
    download: ["pdf", "epub"]
bookdown::pdf_book:
  includes:
    in_header: preamble.tex
  latex_engine: xelatex
  citation_package: natbib
  keep_tex: yes
bookdown::epub_book: 
  stylesheet: style.css