How are PDF sizes specified?

Question

I am generating a PDF from HTML using a library, and all the size parameters that I am giving are in pixels. This seems kind of odd. I just googled the internet for A4 size

Answer 1

I sometimes edit my files in GIMP, when I want to edit my pictures and such. When I export for PDF it depends on the quality I need.

I just use this chart (for A4):