Is there a way to enable opening a PDF with OSX Preview by default rather than the internal viewer?
It would make preferences more cumbersome and reduce the richness of the information content of documents, if they were always opened outside of the DEVONthink environment.
But if you wish to open PDFs outside of DEVONthink, the keyboard shortcut Shift-Command-O will do that, opening the PDF in the parent application (probably Preview) as defined in the Finder.
For example, I prefer reading a PDF such as the DEVONthink Pro user documentation in Preview when I’m looking for the occurrences of a term in the document.
But when I’m doing research, I want to have the document open within DEVONthink’s environment. Within DEVONthink I can compare the information content of a document to other information in the database using the AI assistants such as Classify, See Also and See Related Text. Or I can Option-click on a single-word term and instantly see a list of all the documents in the database that contain that term. I make annotations of my database documents using the Annotation template, so that my notes are searchable and I can link within them to other documents when that’s useful.
In that sense, when I’m exploring ideas, DEVONthink provides a rich environment for that purpose, and Preview feels poverty-stricken by contrast.
Additionally, you can customize the toolbar and add “Open Externally”. I do that because there are a lot of documents I end up opening in Preview. That gives me the ability to view and open windows in DT, but also makes it easy to open in preview.
I too would prefer to have an option for making double-click open in the external PDF viewer.
I appreciate the tips about the keyboard shortcut and toolbar customization for Open Externally, but I’d still prefer the default to open in the external viewer that I use for viewing all my other PDF documents (e.g. those attached to emails and downloaded from the Web). Thanks.
If only the underline color could be changed to red and I could add a note to a highlight, just as easy as I can as in Preview, then I would be happy to stay within the DT context and use its viewer all the time. 
Thanks for explaining the advantages of the DT viewer. Very useful.
I fully expect that users will be asking questions of and thanking Bill for a very long time. As it should be! 
Just read the news. Sorry but I’m speechless for a moment.
Thank you bill, we will miss you.