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Personally I use Latex, more specifically pdflatex, which is free to download. It is text-based, learning it will probably take a week or two, but there are online examples and help through StackEx...
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#2: Initial revision
Personally I use Latex, more specifically pdflatex, which is free to download. It is text-based, learning it will probably take a week or two, but there are online examples and help through StackExchange. The advantage is you can produce charts, diagrams, flowcharts and anything else you want (art for example) using other packages, in their own PDF files, and then "include" them in your documentation. For myself, as a research scientist, some of my graphs can be complex and we use special packages to produce them, and not always the same packages. I might produce a few with just a spreadsheet, and some diagrams with Draw, and others with a specialty package designed to draw, say, molecular diagrams, or engineering diagrams. Latex itself is the best way (by far IMO) to render complex mathematical equations, and to back-reference in the text or build a bibliography. In technical fields (STEM) at least I think this is the package most used in academia and research, in the four universities I have attended or worked at, it is difficult to finish a PhD in a STEM field without being versed in Latex.