I have a 7.8" Onyx Boox Nova 3, which is great for reading novels. The pros with using an Android device is that I can install any reader software I want and combine it with several different dictionaries (I read in several languages, so dictionaries are important to me). The Onyx devices are also very fast and handle PDFs with no problems whatsoever; I also browse the web with it, reading forum discussions, etc.
For your use case, I'd probably recommend a bigger screen. It is possible to read PDFs in landscape mode on the Nova 3, but it's not an optimal experience. It's good enough for me, since I don't read that many and the ones I do read don't contain graphs.
I would recommend you to watch My Deep Guide and his videos about the devices that interest you:
https://www.youtube.com/watch?v=grqBdJ86HWo (Pocketbook InkPad X)
and
https://www.youtube.com/watch?v=EdQ5sXTJRMI (Onyx Boox Note Air).
Personally, I would get an Onyx device if you are reading PDFs. The other readers are just too slow in comparison. Reading PDFs on a Kindle is a nightmare; the Pocketbook seems better in this regard, but still far from the speed of the Onyx ones.
On the other hand they are more expensive. I bought mine from ereader.store when they had a special offer (they have them from time to time - worth checking out).