I don't use my DR1000 as much as I would like to, partly because of the relatively poor battery life. The screen size is about the same as A5 and it's 'okay' for PDF tech books. I have some that includes graphics in colours, and the since the gray-scale dithering on the device is not all that great, the result of this is mediocre. It's best with black and white content. I can't comment on the annotation feature, as I haven't used it.
The DR1000 is an older product and IMHO a little too expensive to invest in now. I don't know how well the annotation feature works on PDFs in the Kindle DX, but I'm generally very satisfied with my Kindle 2, and if the functionality of the DX would be enough for your friend, it might a good alternative. But I've heard the keyboard isn't as easy to use as on the smaller Kindles. You could try asking this question in the specific device forum. There's also the Sony prs-900 to consider. From discussions here, it seems that there really isn't any truly great larger screen readers yet, and any choice would involve a compromise. I came across this recent discussion:
https://www.mobileread.com/forums/showthread.php?t=67986
It really does depend on what your friend plans to use it for and which books he has.