Don't use this, use wkhtmltopdf. Don't waste your life like I did.
Render PDFs using Chrome.
const m = Metalsmith(__dirname)
.use(markdown({
smartypants: true,
gfm: true,
}))
.use(layouts({
engine: 'handlebars',
default: 'default.html',
}))
.use(pdf({
format: 'A4',
printBackground: true,
}))
.build(function(err) {
if (err) throw err
})