Journal articles are usually PDFs formatted for Letter/A4 size, and with rather small type and often in two columns. This makes them almost impossible to be easily readable on 5-6-inch-screen readers. Even in portrait mode the text will probably be too small and an annoying page back and forward would be needed for reading the two columns. It could be a bit better with custom zoom options, but then you'd have the problem of two-column figures, tables or equations.
Devices with a larger screen would be desirable, but those are more expensive too.