For most purposes PDF or other vector graphic formats such as windows metafile and SVG work just fine. However, if I plot lots of points, say 100k, then those files can get quite large and bitmap formats like PNG can be the better option. I just have to be mindful of the resolution.
As an example I create the following plot: x <- rnorm(100000)
plot(x, main=“100,000 points”, col=adjustcolor(“black”, alpha=0.2))
Saving the plot as a PDF creates a 5.