Can any file and its data be converted to a String or plaintext?

can it? Sure, most any arrangement of bits can be converted into some kind of Unicode text. Can it be converted to something meaningful or readable? No, some formats are plain text (.txt, .ini, .json, .html for some random examples) that are meant to be read by humans, and others are binary formats that are only meaningful when decoded by a computer into specific data structures inside a piece of software.