An .sdi file is a plain text file. Writer is not the right tool to create plain text files, though this is possible. The “standard” tool for plain text file is a text editor like KWrite, gedit, Pluma, vi, emacs, NotePad, TextEdit, …
If you don’t feel comfortable with these tools, create the concordance file in Writer. At time of saving,
Save as and select Filter
Text (.txt). Give a name to the file and explicitly force the extension to .sdi.
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