Kobo Aura One with KOreader is good with PDFs. I think it is the best ereader for PDFs, though i haven't tried or looked into the new Forma.
I don't think there is a stylus using eink yet within that size. I've seen this though:
https://www.indiegogo.com/projects/n...hone-monitor#/
https://blog.the-ebook-reader.com/20...-e-ink-device/
I'm also looking for something to take notes with on an eink screen, but I think the closest way or most affordable would be to use a physical keyboard connecting via USB otg. I looked up how people were doing it and saw some diy's with Nook and Onyx. I'd be really happy if someone came up with a way to make a physical keyboard work with KA1.