The PocketBook InkPad Lite (9.7") would probably be the closest you'd come to an eReader that
might work well for PDFs and be "relatively" cheap for its size. It would require dealing with low resolution (150 dpi) but (oddly enough) it does have "warm light" and a microSD slot.
https://www.amazon.com/PocketBook-Gl...s%2C193&sr=8-5
EDIT: I didn't realize until I checked the link more closely that the seller is
not PocketBook and this is
not shipped by Amazon. So be careful. When sold by PocketBook and shipped by Amazon the cost is over $70 more. So this makes me leery of this seller.