[Quoth]

Screen size is pretty important to PDF.
Especially pixels, so a 9.7″ DXG is rubbish compared to an 8″ Sage partly because it's 150 dpi vs 300 dpi.

A TCL Nxtpaper 11 can be got cheaper than any 8″ or 10″ approx eink. About €270 inc the pen.

For just epubs then 6″ or 7″ eink is better & cheaper, though I prefer the 8″ Kobo Sage eink for novels / epubs.

Only the Kobos that officially take a pen, and have a DRM free/unprotected PDF loaded do annotation. Calibre is optional and more value for the epubs/kepubs (real ebooks).