Generally the PDF has better image quality.
So... here's how I convert from PDF. I'm on a Mac, but this is all open source command line stuff, so it should work elsewhere. I use poppler, installed via
Homebrew with `brew install poppler`.
First, I use `pdfimages -all -p` to try just extracting the images. If you're lucky, this will result in usable files. Most commonly, though, you'll get the artwork without the text. Make sure to check a few pages.
If that didn't work, you're gonna need to render the PDF to images. I use `pdftoppm -scale-to <long dimension in px> -cropbox -png` Replace -png with -jpeg if you want smaller files. How big should the scale-to setting be? Well, the size of your device is a reasonable choice. Or maybe the same as the images you extracted with pdfimages. This is gonna be slow. It isn't multithreaded, consider running more than one instance using -f and -l to set the first and last pages to convert.
Is there an easier way? I hope so. But that's what I do.