If an 8-inch screen is enough for you, then I recommend either the Pocketbook Inkpad 4, or the Pocketbook Color 3 if you want color. They run Linux and have a very capable PDF Reader (especially compared to Kindles)
If you want an even bigger screen then sadly they start to get very expensive, and usually ship with an already outdated version of Android and an underpowered SOC. And they also have the terrible standby battery life you would expect from an Android device