How are PDF sizes specified?

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 in pixels, and can I just use these values everywhere?

Is this how it should be done? Will the generated PDF look correctly?

Otherwise, do I need to somehow compute the pixel size using information from the screen?

Then, how do PDF's work if they can be sent to others and still look comparatively the same?