PDF was defined way back before Unicode was ever a thing. It is natively an 8-bit character-set format for text handling. The way it gets around this limit of only 256 characters available is because it also allows defining custom byte to character glyph mappings (think both ASCII and EBCDIC encoding in different parts of the same document). To typeset a glyph that is not in the current in use 256 character sub-set mapping you switch to a different custom byte value to character glyph mapping to typeset that other character.