I am very happy with my Kobo Elipsa (https://us.kobobooks.com/products/kobo-elipsa).
I use the following flow very often: + Place a PDF in dropbox folder, sync to Elipsa + Read on Elipsa, annotate in various ways + Sync back to dropbox. + Look at PDF on my desktop computer (linux), annotations visible. I use okular to read PDF on linux.
