$300 dollars budget you say? You might want to consider this
https://www.pocketbook-readers.de/de...x/sArticle/182
The screen is the same as Kobo Aura One, but everything on board is better. For example the equivalent of ComfortLight is there, even though Pocketbook does not really need it, because its light is somewhat yellowish, not evil blue as in Kobo.
Every Pocketbook has the best software for technical/huge pdf files in out-of-the-box/unconfigured state. It can zoom in to any part of the page by pinch zoom and you can drag the zoomed page around in any direction like on a tablet (Koreader cannot), crop to decrease margins (Koreader can do this too), and it has automatic multi-column mode for multi-column documents (Koreader only has it for two columns).
In Pocketbook Inkpad 2 (unfortunately not there in Inkpad 3, as far as I can tell) there is also an ingenious annotation system. When you annotate or highlight something in a file, it is reflected in a separate Notes app. Open the Notes app and you see the list of annotations for your entire e-library and by a tap you can instantly go to the relevant book, the spot you need! This can possibly be emulated somewhat in a raw form in Inkpad 3. If this feature sounds something you might want, look around for a used Inkpad 2 (8" screen with a bit worse DPI).
Kindle or Kobo cannot match these features. They are evidently not even trying to make a tool for serious researchers. Installing Koreader on a Kindle makes you lose your warranty. In Pocketbook you don't even need to install Koreader in any dangerous sense - it's just a matter of copying a folder in the right place, it can be tracelessly deleted, and the warranty stays intact.