Colin Gillespie shows us how to change PDF and PNG output settings within knitr:
In many workflows, function calls to graphic devices are not explicit. Instead, the call is made by another package, such as knitr.
When kniting an Rmarkdown document, the default graphics device when creating PDF documents is
grDevices::pdf()and for HTML documents it’sgrDevices::png(). As we demostrated, these are the worst possible choices!
Click through to see what you can do about it.