It's not just a matter of screen size, it's the nature of eink for the task, as well as the software.
If you've heard that Kindle's handle PDFs badly, it primarily dealing with trying to reflow text PDFs, so that one can smoothly read a novel.
Oddly, the smaller 5" Sony PRS-350 handles that sort of PDF reading better than most of the 6" inch devices.
But, the larger DX should be among the best for your needs, as you'll be viewing an entire page of the PDF with out trying to reflow or handle graphics and tables, so you won't be running into the areas where Kindle's PDF handling is lacking.
I suspect any other 9" or larger eink device would work for you just as well, but I have no experience with any of them.
Using a 6" or 7" eink screen in landscape mode may be an option, if you don't mind that constant half-page scrolling. I've not spent time doing such a thing, so I'd just be speculating.
If at all possible, order from a company with a liberal return policy so you can try it without much risk.