The description of ltr format expansion
The ltr file is a letter file, which is being used by a wide variety of document authoring programs. The ltr file contains a message or other correspondence. The format of data, being stored in such a file, is plain text.
Nowadays all ltr files have become obsolete, as far as most of the apps, which were capable to create those files, have been discontinued. Nevertheless, if you encounter such a file, you will easily be able to open it.