Every page is rendered at double size
Each page is drawn to an image at twice its natural dimensions, which works out to about 144 pixels per inch, and saved as a PNG.
A PDF measures itself in points, at seventy-two to the inch, so rendering at twice that scale gives roughly 144 pixels per inch. A standard A4 page comes out around 1240 by 1754 pixels. That is sharp enough to read comfortably on a screen and to paste into a document or a slide, while staying well short of the very large files a 300 dpi print render would produce. The scale is fixed, so there is no quality or resolution setting to choose.