I’ve posted about this before. I was advised to uncheck the decluttering option, and to wait for the next maintenance update. I’ve done both, and increasingly the problem is getting worse. The only way I can read pdfs in DT is to open them in the original application.
iMac MacOS 10.12.6
DT Pro 2.9.17
So the PDFs can be displayed in other applications but not in DEVONthink? This is a random issue of Sierra’s PDFkit, you could either upgrade to High Sierra (recommended) or enable a workaround (see hidden preferences in help).
I’m curious, what is the “original application”?
What are “hidden preferences in help”?
Choose Help > DEVONthink Help, enter “hidden preferences” (without quotes) in the search field and select the result “Appendix: Hidden Preferences”.