A PDF can include the fonts. But it often doesn't, and relies on system fonts. One reason for that is because including fonts in the PDF can dramatically increase the size of the file. In some cases a single font could be larger than the entire rest of the file. I've also worked on implementing embedding fonts in some software that generated PDFs. It was surprisingly difficult to figure out how to get it to work reliably.
> PDFs are rendered consistently.
Not as much as you would think. There are several cases where the same PDF will render differently depending on which PDF viewer you use. Usually the differences are pretty subtle, but occasionally there are edge cases that result in pretty significant differences. I've even run into a case where the same version of Acrobat reader will render a PDF differently depending on what OS you are using.