Why a scanned PDF reads so badly on an e-reader
A scanned PDF has no text in it, only pictures of pages. On a 6-inch screen you either zoom and pan across every page or squint at a page shrunk to fit. You can't change the font size, search, or look up a word.
Many converters that accept scans just wrap those page images in an EPUB file. The file is called an EPUB, but it still reads like a photocopy.
