When the audience you are writing for will not be able to retrieve the source you have used, it should not be included in the reference list. You can either treat the work as personal communication or, if the document is important for your assignment, you can add it as an appendix. Don't forget to ask for permission from the company or organization behind it first. The appendix is not included in the reference list and is only cited as an in-text citation.
This is described in the guidelines for Region Jönköping (see Appendix).
If you have several appendixes you name them with capital letters in the same order they appear in your text:
This is described in the guidelines for Region Jönköping (see Appendix C).