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E-book Reader Matrix has a row on PDF Capabilities. Almost all readers that use mobile Adobe Digital Editions include "Single Page Reflow" which treats each page of the PDF like a text-based web page. The 6" EInk reader with the best PDF support (reflow and zoom) is likely the Sony PRS-T1.
The advantage of a tablet with a LCD screen (Fire or Nook Color/Tablet or Kobo Vox) is that pan and zoom can be faster. On the Fire and Vox you will be able to install 3rd party apps, including PDF reading apps, although the Fire might (or might not) be limited to apps from Amazon's own app store. The Nook Color can be jailbroken to allow 3rd party apps. I don't have any experience with how well a 7" tablet screen works for PDFs with the available apps.