Wayne, the File > Link to command was removed in version 1.1 or 1.1.1 and we haven’t yet gotten around to removing all references to it.
Using File > Index produces the same result. You can then create a text link to the Indexed file.
Actually I prefer a different and more direct workaround. In a rich text document, place your insertion point where you would like to insert a clickable link to the external file (be sure to have an extra space or punctuation mark to the right of the insertion point, so that you can continue typing without “extending” the link as you type).
From the Finder, select the file to be linked to and Option-Command-Drag & drop it to the insertion point in your DT Pro document window. Now you’ve got a clickable link to the file.
If you wish, you can select the link string and type different text over it. The link will still work. Example: change “filename.txt” to “see Jones et al”. How’s that?
If you wish to Index (link to) or Import (copy to database) unknown file types such as a PowerPoint file, you need to set your preferences to do so. Otherwise, they will be ignored.
Select in the menu bar DEVONthink Pro > Preferences and click on the Import tab. Under File: select “Unknown file types”.
Then you can index or import, e.g. a PowerPoint file. A blank document will be created in your database, as the text cannot be read by DT Pro. However, you can add searchable text in the Info Comment field such as a summary or description of the presentation.
And the log will report an unknown file type.
Remember that you can “print” any printable document to DT Pro as a PDF. So from PowerPoint you can select File > Print. When the Print panel appears click on the PDF button, then select “Save to DEVONthink Pro.scpt”. You will be asked to choose the group location to which the PDF will be sent. The PDF file itself will be stored in your database Files folder. And you can search and analyze the text in the presentation.