How I can generate documentation for unty C# code so I can later copy it to pdf file
Check out Doxygen.
Type /// before each class or function. Monodevelop will display your documentation in the tooltip/code-hinting/code-sense. Doxygen can read this documentation and make nice HTML, LaTeX, or Rich Text that is easily convertible to PDFs.
I’ve used Sandcastle A few times, never let me down, so i’ll recommend it for sure, it’s also free and pretty straight forward! 
thx for help, can I describe variables the same way?