KRA opens only in Krita, and the layers are meaningless elsewhere. PDF looks the same on every device and keeps several pages in one file. KRA is at home in digital painting that is still being edited; 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.
The picture is the flattened image that Krita stored in the file, so the layers are not kept and cannot be edited afterwards. 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.