I highly recommend looking into Kobo. They sell ereaders with a Linux OS base and work very well with the Calibre eBook management software.

I like Boox but they are on the more pricey side. Basically an android tablet with an e-ink display.

I have sung praises of kobo ereaders many times before. You can go to the search and look for comments made by me that say "kobo".

I don't know any the new models though so check on that. The gist is that they are not locked down so you can read pirated ebooks on them.

I'm not sure about PDFs. Because of their nature I've not tried to read them on an ereader.

Go on ebay and grab a Kobo Aura. Then download Calibre to your computer. Then go to Libgen for any book you want

Kindles suck thanks to Amazon lock in, but I pirate all my books and use Calibre to convert/manage my library, and can easily use email to send books to my device. The main reason I stick with this one (paper white) is the battery life. I go a month or two between charges, reading heavily. My wife’s Kobo needs to be charged every couple weeks and she’s a much lighter reader than me.

I would suggest a Kobo e-reader, their cheap model is perfect imo.

If you would like to add more options to the software, there’s open source software you can easily side load called “Ko-Reader”, navigating pdf files is much easier than stock for example.

As far as walled gardens go, you can easily sync ebooks with your computer using Calibre to manage your library. You can also add books when out and about using your phone as a mobile hotspot if you enable the beta web browser on the kobo and use https://send.djazz.se/

Editing to add: I always either convert my pdf’s into epub’s using calibre or pirate the epubs directly, there aren’t many e-readers that do PDF’s well, and those are all pretty expensive. Things like the Onyx Boox or Sony’s Digital Paper Series

PDFs are hard because you need a big e-reader screen. Text only formats like Epub or Mobi are much easier. If you think you can find epubs for all the PDFs you want to read, I'd recommend getting an old Kindle. I'm partial to the Kindle Touch (which can be jailbroken easily). You can often find them for around 20 USD on eBay, which is the cheapest you can get something like this.

what sort of PDFs do you need to read? are they comic books, illustrated magazines, scientific articles? do you need to take hand written notes or do markups?

I got a kobo after a recommendation here, and it reads pdfs fine enough for me

I also have a Kobo :) I think a Clara? It was pretty cheap, but I can’t email files to it, have to transfer via USB

The boox readers appear to be the best because they have a full android rom, but they're expensive. My experience with Kobo has been bad, I've had 3 devices fail for various reasons even though they're quite a bit more expensive than Kindle.

I've since switched to a refurbished Kindle Oasis, which is a great piece of hardware for the money. Top notch screen, light, great form factor, real buttons that are well placed. It's got the stupid Amazon ecosystem, but I don't pay for books, so it doesn't bother me, I just don't use it. They still give you an email address so you can easily send books from calibre without needing to plug it in.

I had a kobo Clara colour but broke the screen (no idea how).

I replaced it with a Book poke 4 lite and that's better IMO, but the kobo was good.

Ideally you'll want one that isnt locked into any ecosystem so you can do what you want on it.

I got a Kobo Libra and immediately put KOReader on there. Works great and jailbreaking is extremely easy.

I uſe a Kobo Libra 2 with KoReader, and Calibre to edit and convert e-books. It’s not as large as ſome other models ; ſo it wouldn’t be as well-ſuited to PDFs or certain textbooks, but as as been ſaid elſewhere in the thread, it’s not unreaſonable to convert thoſe to e-pub or mobi for a better experience. What it does have as a benefit over ſome other e-readers at the moment is a higher contraſt ratio and generally good reſolution—not quite as nice as a print book, but very cloſe. The ſcreen itſelf ends up being cloſer to a newſpaper in print quality than a nice book due to the tint and general contraſt, but this can be a bit leſs apparent with brighter lighting or direct ſunlight. It’s an overall nice experience and can act as a replacement for phyſical books, but both methods have their advantages and diſadvantages.