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.
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.