.ZRTF File Extension

To create a ZRTF file, select File → Save As... or Save Copy As..., name the file, choose the save location, select "Nisus Compressed Rich Text" from the "File Format" drop down menu, and click Save.

To open a ZRTF file, simply double-click the file, or select File → Open..., navigate to your file, and select Open.

NOTE: The ZRTF file is saved in a proprietary format but if you rename the file from filename.zrtf to filename.rtf.zip, the Finder will expand the file into a regular RTF file.