I just upgraded to version 1.9. It appears that all pdf images imported since the upgrade are low resolution. They are unreadable when zoomed. Images imported before the upgrade look fine.
I just checked a new PDF import and it looks fine, and is readable when zoomed.
What are your preference settings for PDF import?
Could you try reimporting a previous PDF file and check the results?
Old PDF’s had acceptable quality. I loaded 1.9a and the problem appears to be corrected.
The problem of low resolution pdf files came back. I checked my preferences and the create a thumbnail button was checked. I unchecked it and the problem has not reappeared. Does that check box cause only a thumbnail of the pdf to be stored in Devonthink?
No, it just creates a thumbnail icon for the PDF, nothing more. Maybe some sort of a Voodoo effect 
Best,
Eric.