When I use the render{rmarkdown} to produce pdf file from .Rmd file on my Mac, an error message says
pandoc: pdflatex not found. pdflatex is needed for pdf output.
For people using ubuntu who get stranded here a better option (because it is 1/5 the size) is to use:
sudo apt-get install texlive-latex-base
Which I found via https://tex.stackexchange.com/a/128309