Usually it is the PDF that is at fault, or rather, the assumptions made by the author (or the authoring software). A PDF meant to be portable and usable for a long time should embed all fonts used. If you get different fonts and sizes in different viewers, that tends to hint at a lack of embedded fonts, leaving the PDF viewer to substitute using its fall back fonts.

Most PDFs I come across tend to come with embedded fonts and work just fine on any decent PDF-viewer.

If the underlying document doesn't embed the fonts it uses, then the reader would perform substitution. This causes different outputs on different readers.

It's more of a PDF producing application problem that fail to either embed the rather uncommon or proprietary fonts.

That's why when you're trying to print professionally, you're gonna use the original software and not something that try to re implement it. I know it's sad, because open source, but when a (good) standard exist that is closed source, we have to made due, till something arguably better exist.