How to save PDF so actual size is the same as artboard size?

I created an artboard that is 1416px wide.

I need to handle the PDF to a coder, with it having the same actual size as the website.

Yet when I save it as a PDF and he gives it a 100% zoom (chooses view>actual size) it appears a lot bigger than what I intended. I checked the file dimensions and it stills says 1416px wide, also if I export it for web as a JPG and compare the two the JPG has the correct size while the PDF is bigger.

JPG Actual Size

PDF Actual Size (A lot bigger)

Why could this happen?