If I’m to choose myself, I prefer the standard tools: Word, PowerPoint, Visio, Adobe Professional. When the project is finished, I give you the editable files, so you can keep them up to date yourself. I’ll be happy to give you the necessary explanation on the templates I made. They will make it easier for you to maintain your documentation.
In some cases I recommend the use of a specialized tool such as FS Pro or manual.to. In such case, you need to purchase the tool yourself and I can assist you in the use of it.
A third option is that you already have a tool at your disposal. Normally it is no problem for me to write in that tool.