[MarjaE]

It depends what you're reading.

For works in epub or mobi, any reader can work, although some books are licensed for specific readers. You can use Calibre to convert epub to mobi or vice-versa. You could use add-ons to transfer licensed books to different devices.

I find it easier to navigate a lot of titles, organize them into collections, and move them into special collections once read easier on the Kindle, but handling large books easier on the old Story Hd or on tablets.

For works only in pdf, then you have to be more careful. If you have really good eyesight, then a small screen with high resolution might work, but if you have poor eyesight, then a larger screen might be necessary. I have more of an astigmatism than my glasses can correct for, so while I can use a 7.8" screen by squinting or by roating everything and using multiple flips for each page, I have far easier time with a 10" screen.

Scanned books are usually in pdf, with much worse epub, mobi, and text versions. Articles are usually in pdf, though pubmed often has epub versions, and dot epub may be able to create epubs from web versions. Game rules, art books, atlases, and so on are also usually in pdf.

Newer pdfs are often incompatible with older readers. Unless you want to pre-process them for your reader, you may want an e-ink tablet instead.