I was under the impression the smartphone programs either stripped out art or it shrank the art down even further until it's impossible to see.

Sometimes some funky things happen with artwork (turned upside down, or it doesn't format properly) but most often it just shrinks down. If it's a large banner it's usually still visible, but the smaller ones do indeed become too small to pick out clearly. Mostly, I suspect it doesn't often get placed in the right spot if it's a PDF conversion, especially because I enlarge the font to the max so I'm not really looking at "pages."