Storing: filesystem (notes include where I stored it).
Reading: E-ink.
I started with Pocketbook 622 (a 6", 800x600 display). Worked very well. Can open many formats _natively_ (doc, rtf, djvu etc, check specs for full list). One of the first docs was anatomy atlas from 19century via archive. Rendered only decently, required huge magnification/landscape mode/margin cutting to be of any use. I had varying experience with other pdf/djvu documents - depending how they were created. Some djvus rendered excellently on 6", despite being meant for bigger (close to a4) page size. No problem with rtf/epub and other such formats. Magazines in pdf (a4) very hard to read, not worth it really. Arxiv's pdfs looked good/very good, sometimes they could be reflowed or put into column view, which helped a lot but with reflow I learned math not always shows up properly. Old computer manuals (my hobby, they are just scaned typewritten books) - not good enough.
Next model was Inkpad 840 (a 8", 1600x1200 display). What looks good on Pb622, looks good too on Ip840. Magazines look better, but they require a good light for really comfortably reading. Otherwise, I can go with dim night light. This model has backlight, but I don't like the idea of shining into my eyes.
Huge plus: sd card slot. I go on for months airgapped. Huge minus: maybe it is just me, but reading html docs almost always sucks one way or another. What to look for: external hard case so I don't have to be oh so wary. It was a PITA trying to find case for Ip840 thanks to its nonstandard dimensions. I settled down with some oversized tablet case. Ip840 feels a bit slow and awkward (compared to Pb622) but I got used to it. If I had to buy again, I would have had a closer look on Kobo models too. Kindle does not cut it for me - requires too big commitment.
All of this just MHO, of course.