In DEVONthink, if a type of file can be edited in DEVONthink, it will open in a DEVONthink document window when the file is double-clicked. This makes it easy to isolate a document for editing while allowing you to keep your place in the other window. It also allows you to keep working directly in DEVONthink without having to jump back and forth between applications.
If a type of file can’t be edited in DEVONthink, double-clicking will open the file in the system default application. For example, you can display an Excel spreadsheet but can’t edit it. Double-clicking will open it in Excel. Make you changes, save the file, and when you return to DEVONthink, the changes should be displayed.
In the case when you have a document that can be edited in DEVONthink but would like to open it in the default application instead, you also have the option of pressing Shift-Command-O. Or you right-click the toolbar and add an Open Externally button. The name of this button will dynamically change, depending on the type of file selected.
(The question will arise: Can I change this behavior? The answer is: no. The behavior was changed some time ago in response to users’ desire to work within DEVONthink as much as possible.)