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For documents that DEVONthink can't edit itself—or in cases where you need special features found in a different editor—you can open any document in another application of your choice.
If you double-click a document and DEVONthink can't edit it, it automatically launches the default application for that document type and opens the document. (This happens, for example, with Excel and Pages documents.) If you don't want that to happen—you want DEVONthink to open the document in a separate window, even though you can only view the document there—instead select the document and choose Data > Open (Command-O).