The following should be a maxim (aka fundamental rule) in every software project:

If it’s not under version control, it does not exist.

One way which helped me several times is the following:

  1. When you release your software (in continuous deployment scenarios, you might stick to major releases…), create a pdf file from your documentation.
  2. Put this pdf under version control, in the same branch, tag/label or release info as your code-to-be-released.

Don’t accept a lack of version control!

Really (really!) make sure you find a (mostly automated) way to get your documentation under version control.