PDFs don't always lay out characters in sequence, sometimes they have absolutely positioned individual characters instead.
PDFs don't always use UTF-8, sometimes they assign random-seeming numbers to individual glyphs (this is common if unused glyphs are stripped from an embedded font, for example)
PDFs don't always use UTF-8, sometimes they assign random-seeming numbers to individual glyphs (this is common if unused glyphs are stripped from an embedded font, for example)
etc etc