Sony's 13" PDF reader has just gotten a little cheaper.
That's a significant discount from the original price, but still a little higher than the Onyx Boox Max, which is selling for 585 Euros. The Boox Max is the more capable device thanks to running Android, but given how the massive screen limits what you can do with it I don't think that matters.
The Sony DPT-S1 only supports PDF, and its dual touchscreen tech lets users annotate the files with a stylus or with their fingers. It has Wifi, 4GB of storage, a microSD card slot, and according to user reviews it's light and flexible, and makes for a great cutting board.
I've never had a chance to really review the DPT-S1, and after having had my hands on the Boox Max I have no interest in either device. They're much too large to be used as an ereader, and I have no need for a PDF reader.