The document provides instructions on modifying font and font size in R graphical parameters using the plot() function. It explains how to customize the fonts for titles, subtitles, axes, and labels, as well as the corresponding arguments (font.main, font.sub, font.axis, font.lab, cex.main, cex.sub, cex.axis, cex.lab) and their value ranges. The document also includes example code for each modification along with visual representations.