An A4 PDF will not display well on any 6" device.
To be honest with you, the best device currently on the market for displaying and annotating PDFs is the Apple iPad. The Kindle DX will display them OK, but it currently has no annotation capabilities for PDF files (although this will probably change in a future firmware update).
If you're looking for a device now though, I'd go for the iPad (if it's within your budget, of course).
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