Because I don't want anything to try to reflow the text, or adjust the kerning, or modify to use system fonts.

There are great systems for those already.

When I want a PDF, it's because I want a format that I know is always going to look the same.

A PDF is a great archive format. It's perfect for a scan of a document, or a printout.

I never want my viewer to add anything to it, I never want it to detect anything, I never want it to adjust anything.

Just render it exactly the same way, every time.