For documents containing nothing but character representation (basically text and numbers!) it's possible to use a standard Text-Editor, a program that's usually included in every operating system. 

A list of text editors can be found on Wikipedia.

In documents like this there is no formatting whatsoever. This can lead to a bad readability. Therefor Plain Text is only recommended for very short documents like E-Mails or short abstracts.

For other documents it's better to use a Word Processor Program like Libre Office. 

Users can benefit from the Interface with many different functions to design the layout of their texts. It's possible to insert tables or bring out parts of the text. It's also possible to insert pictures or other objects. It's important to save and archive the other objects separately. 

It varies in what format a text is filed, depending on the Word Processor Program in use. (default .doc/.docx in MS Word, .odt in Libre Office)

In terms of long-term preservation it's best to use the benefits of a Word Processor Program, but file the document as PDF using its "Save as..." function.