Every day I'm more sure that the best way to maintains a company documentation across time is using plain text documents. Specifically, I'm using the markdown format to manage all the documentation of my company, and the documentation of some of my customers.
Benefits to write in plain text
Everyone can open them
You don't have to worry about the people can open the file. Who don't have been has problem trying to open a .docx document file on older version of Word. All people have an application that can open a text file.
No format problems
A lot of times we have problems or spent a lot of time formatting a document. With plain text document we never have this problem, a plain text document always looks the same on any system. And using simple formats like markdown we can add the most used formats like title, lists, tables, etc..
We don't have problems when copy and paste a text in a document.
The plain text editors are simple, not have a lot of options that distract you when writing. On the other site, plain text force you keep things simple.
The plain text documents are light. You can edit plain text documents from old computers or low power computers, and you can sent them by emails without be worry about the size.
You can add them to any version control system like git or subversion to control the document changes. This is very useful to follow changes in the document, and add notes to this changes. This is a typical practice for software developers that we can use to manage the company documentation.
There are a lot of tools to convert or export the plain text documents to another formats (HTML, Word, PDF, Ebooks, ...). Pandoc is an example.
Easy share with the team
Plain text documents are specially easy shareable when you use them with git or other version control system. Every user can have a local copy, without depend of Internet connection or a central repository. You can modify the document and upload them to git when finish.
Easy to learn
Because the syntax of plain text format is very simple, the time to learn how to write documents using plain text syntax is very short.
Markdown is a lightweight markup language with plain text formatting syntax, invented by John Gruber. It is designed so that it can be converted to HTML and many other formats using a tool by the same name.
Markdown’s syntax is very simple, and cover the basics to write company document: titles, lists, code blocks, links, etc. And if you need more, you can expand the basic using some extensions, for example to add tables.
Friendly markdown editors
https://atom.io/ Windows, Mac and Linux.
http://www.texts.io/ Windows GUI editor. $19 per user.
http://pad.haroopress.com/ Windows, Mac and Linux.
https://typora.io/ Windows, Mac and Linux.
https://caret.io/ Windows, Mac and Linux. $29