i have the same use case (technical PDFs) and I use the original iPad pro (12.9"), the stylus is great for annotating PDFs, I use the GoodNotes app, it's fantastic, remember the resolution is high, 264 ppi. Reading on computer screens with the old 96dpi was a common source of eye strain. There is a newer iPad pro now too but if you are set on e ink then avoid goodereader at all costs! Consider the Onyx book max 2 pro (13.3") or whatever it's called now and the Sony DPT-RP1 (13.3") or wait for the new model which is rumoured to come out this year. If you are happy with a smaller display size, which i have not tried myself, but might be fine, there is the Sony DPT-CP1, the Boyue Likebook Mimas, the Onyx Boox Note Pro or this E-Pad (looks interesting)
https://www.kickstarter.com/projects...android-tablet