The default font size in rst2pdf is just 10pt. This results in very long lines, that can be very difficult to read. The common¹ acceptable range is 45-75 characters, with 66 being the optimum. The PDF currently has around 100 characters. Changing this is unfortunately a lot of work, as many things in rst2pdf's styles don't automatically follow the font size. ¹ https://en.wikipedia.org/wiki/Line_length
Since this is about readability, it might be worth also changing from justified text to left aligned.
For reference, our DocBook-based PDF had similar line length. It had left aligned lines, though, not justified.
