Good E-Ink Based E-Reader for Technical PDF Docs

The market leader in that category is http://www.sony.com/electronics/digital-paper-notepad

However it is pricey!

Even though it is listed on Sony's website, none of the vendors on Sony's website has any in stock, and CDW lists it as discontinued as of January 2017.

(I checked B&H, Amazon, and CDW.)

Kobo Aura One https://us.kobobooks.com/products/kobo-aura-one

Not sure how well it works, but have you tried converting the PDFs to the Kindle native format for more native text flow?

https://getremarkable.com/ This appears to be a good option but they are still in preorder.