And PDFs can be created specifically for a 6" screen. Of course, most PDFs you find around are probably created with US-letter or A4 page size, but it doesn't mean that is inherent to PDF, you can create it with any page size you want.
Or put it another way, take your preferred ebook format, your preferred reader, font size, layout, etc... A PDF can be made to look exactly like that. The drawback is that it will often not be reflowable, but it need not be less readable than other epub formats. (Now conversion is a different matter, that will always be a PITA.)