I think as far as e-Ink devices the KDXG is probably the best current model device for PDFs. No e-Ink device is going to do a great job with PDFs compared to a computer or even the iPad. The e-Ink devices tend to have slower processors, and e-Ink screens are less responsive (slower) than LCD screens. But the iPad and computers have limitations as well. They tend to be bigger or at least heavier, and the LCD screens are more of a strain on the eyes for prolonged reading, and at least in the case of the iPad are too glossy and reflect too much background light. Their battery life is usually less too.

With the laws now being interpreted (in the USA at least) that it is legal to jail break an electronic device, that might open up third-party development. Amazon, and even greedy Apple, are not likely to pursue individuals in legal battles when it will cost them a fortune and they cannot get much as far as a settlement. Developers who try to make a living off of third-party jail-break software might be targeted, but those who do open source development are probably fairly safe. I wouldn't worry about Amazon as they seemed to be committed to using multiple platforms and to selling books. Apple on the other hand wants to control both hardware and software, so they might be more apt to go after developers, even if they are doing open source software.

I think with companies like Amazon, they might even gladly look the other way if open source enhancements greatly improve their readers and they see an increase in book sales as a result. After all, books sales is where their "bread and butter" profits come from. Apple on the other hand is more controlling, but since they don't make e-Ink readers that is a moot point.