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.