I guessed this is a type of XSS but it seems not. The TL;DR is a bit vague on the impact. It says "This bug allows an attacker to execute arbitrary JavaScript code as soon as a malicious PDF file is opened" but PDFs can already execute arbitrary JavaScript as a feature (as noted in the article).

Hidden in some paragraph it does say

> Instead, PDF.js runs under the origin resource://pdf.js. This prevents access to local files, but it is slightly more privileged in other aspects.

Seems like it's not an XSS letting you take over the website origin, but it lets you run JS under this resource://pdf.js origin. Could be an interesting vector when combined with other weaknesses, but not an instant knock out as I expected when I read the title and saw the points :)