What do you mean "at least not on pdf files"? PDFs are the only files that the zoom option is available for. The option isn't there for the Kindle's own format (it's not required), so it can't have worked for you on any other file type.
Yes on a normally kindle formatted book on a PDF it's the zoom option instead of change text size. Not a PDF you have used caliber to convert for your kindle a actual PDF file
