There is a long-standing bug in the Webkit engine used by this class, which means that sometimes the text in the resulting pdf is rendered poorly when using small font sizes (e.g. <= 9 px). The kerning on some of the characters is wrong.
This can be fixed by forcing the engine to include the font within the generated PDF.
Simply put, include a font within your html to be converted using the font-face CSS rule. E.g.: