In the Properties inspector for a document, there is a “Comment” box:
What is the PDF property to which this “comment” field is mapped? (I would test it, but I can’t seem to edit the field from within DEVONthink.)
None. That’s the reason why you can’t edit it.
Checked and yes, it’s not available for PDFs:
Also see this old blog post, which is mentioned in this thread.
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Thanks for the fast reply! I did actually search the forums but apparently didn’t go back far enough. Anyway, this explains the enigmatic comment box.
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The “trick” is to search for “PDF comment” and one of the other properties. With “pdf comment subject” and sorted by relevance Bluefrog’s explanation is the fourth result 
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