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 that get stranded here because the same error is showing up in their Linux distribution. Get pdflatex in e.g. Ubuntu by installing
sudo apt-get install texlive-latex-extra
its too much software, but needed for example by knitr (rmarkdown-pdf-compilation)