DBF Manager™ - Work with index files
How to open a .cdx, .mdx, .ndx, .ntx index file
The .cdx, .mdx, .ndx, .ntx index files contain and maintain pointers to records in the table (.dbf) file and is organized according to the index key values. An index file is separate from but associated with a .dbf file. So, you should open a .dbf file firstly and then associated index file(s) if needed.
Unlike .cdx/.mdx files that open automatically when the associated table is opened, you must open a .ndx/.ntx file explicitly using program's Index Manager dialog box (File->Index manager menu item).
Index files are only relevant for searching in the DBF - you really don't need them (i.e. you CAN read the .dbf data file without the index file). Removing the index file might break you application though.