A "readable font" refers to the style and design of the letters that make up written text, which are easy for people to understand and read easily. Full definition
Since, this is a professional letter; it must be drafted using professional font style and readable font size. (bestcoverletters.com)
Use readable fonts for body text and headers and include some white space. (resume-now.com)
A well - organized format includes using an easily readable font, providing plenty of white space and organizing sections logically so the reader doesn't have to hunt for important information. (monster.com)