ICNS opens only on a Mac and in icon tools. PDF looks the same on every device and keeps several pages in one file. ICNS is at home in macOS application and folder icons; PDF is the usual choice for documents that must look the same everywhere, and printing.
Add several files and you can also combine them into a single PDF. The defaults suit most files. Under Options you can choose the page size.
If the icon file holds several sizes, the largest one is converted. Each picture becomes one page. The page can match the picture's own size or be A4 or US Letter; on paper the sheet turns to suit the picture, which is shrunk to fit inside the margins and never enlarged.