I use Pocketbook 301+ with technical math books and research papers all the time, and I am very happy with it. It has by far the best PDF implementation from all 6" devices I know.
- Comes with a great case with a nice leather book look
- 6 inches is a great compromise between size and portability (I feel 8' might feel too bulky to carry around)
- Excellent two column mode for A4 and letter papers
- Crop margins option in PDF that really works
- Arbitrary persistent zoom
- Two PDF viewers
- Most books very readable in landscape mode, some even on portrait mode
Concerning PDF: on 6" devices it's all about the software. Before the Pocketbook I had a PRS600 that I returned after two days. The glare on the screen was too much for me, and the absence of an autocrop margins option was a deal breaker.
Good luck in your choice.