If you are *only* doing pdfs then your best bet is one of the open Android-based ereaders. Then you can install Adobe reader, Repligo, or whatever PDF app you prefer.
Alternately, if you don't mind LCD screens, a small/cheap Windows 10 tablet will do tbe same and give you color.
Edit:
For that matter, Sony has two eink tablets designed expressly for PDF reading and annotation. A bit pricier than the eink Android readers but also much bigger. And with PDF, screen size matters.
https://www.sony.com/electronics/dig...ads/dpt-series