Here's the text from that PDF:
"Canvas fingerprinting is a type of browser or device fingerprinting technique that was first presented in a paper by Mowery and Shacham in 2012. The authors found that by using the Canvas API of modern browsers, an adversary can exploit the subtle differences in the rendering of the same text to extract a consistent fingerprint that can easily be obtained in a fraction of a second without user's awareness.
"The same text can be rendered in different ways on different computers depending on the operating system, font library, graphics card, graphics driver and the browser. This may be due to the differences in font rasterization such as anti-aliasing, hinting or sub-pixel smoothing, differences in system fonts, API implementations or even the physical display. In order to maximize the diversity of outcomes, the adversary may draw as many different letters as possible to the canvas."