[Quoth]

You need a 10" or larger tablet or eink for PDFs. There is no 100% reliable way to convert all PDFs. Many would need OCR and proofed. A lot of work.

The 10.3" Kobo Elipsa can highlight scanned PDFs. Probably all large eink can. Some can do notes with Handwriting recognition, but not on the PDFs. Usually you can "write" on the PDFs with any larger eink or Tablet (10" or more) that has a true digitiser stylus/pen/pencil, but often not if the PDF is locked by DRM. If you can "export" the Annotated PDF at all, you just have a PDF that looks like a scan of a paper document written on with a ballpoint pen. Usually no text export of annotations, that mostly only works with real ebooks (epub and proper reflowable Kindle formats).

PDF ultimately is a read-only fixed layout format intended to preview print, source for print or WYSIWG fixed size/layout to read only on screens large enough for the defined "paper" size.