I've recently been asked to create documentation for a solution we're currently working on. In the past, when the client has been wiling to fund such an endeavor (documentation, especially good documentation, is very time consuming e.i. expensive), we've used actual documents, PDFs, a knowledge-base table in the file, and in the past few years, a wiki.
I'm curious to know what others use and what other options might be out there to explore. We settled on wikis as they are easy to set up, provide basic security, and allow text, images, videos, and other files formats included inline or as attachments. We can create pages as we code so instructions tend to be task-specific and in smaller chunks of data, they are easier and quicker to create. On the plus side, the documentation makes a nice reference guide for us looking back over the years as to what we did and why. Most of our solutions are in use for many years and we can't remember everything.
Looking forward to what others have to say about this.