What is 100% zoom in a PDF?

When I open a PDF and set it to 100% zoom, what exactly does that mean? 100% of what?

If I have a landscape letter size page, 100% zoom goes beyond the visible Reader window.  I am supposing this has something to do with document and screen resolution, but PDF Properties doesn't list a document's resolution, only it's size.
Related to this, if I want a document to open at 100% and give a specific size, say 1920 x 2400 pix, how would I go about achieving that?

Thanks.