itextsharp: what is the height of a regular PDF page in pixels?

What is the height of a regular PDF page in pixels?

I heard it was something like this:

Dim pgSize As New iTextSharp.text.Rectangle(595, 792)

but I am adding an image that takes up maybe half the height, and even though pgSize looks like a full page and the image takes up only half of it, I am getting a height of like 619 for the image?

I do not know if it is in the same units?