Does anyone know if Notepad++ can display a vertical line on the right margin to indicated that the text will or will not fit on a printed page? I have looked but I cannot find it. Word wrap does work, but it wraps by my screen width, not by the width of a printed page.
As you said you do not actually want to print it, you may use the Show vertical edge.
Setting > Preferences > Editing > Tick "Show vertical edge" > Choose "Line Mode"
You can then specify the vertical line should locate in which column (default should be 100).
(Tested in: Notepad++ v5.7)
BTW You can also customize the color of that vertical bar here:
I think modern Printers are able to auto adjust words so that can fit in the sheet. So don't worry about margins.
I suggest you use to Microsoft Word for typing documents that you want to print.
EDIT: I couldn't find anything that could provide a vertical margin in Notepad++. But as Workaround for text readability, once the text is written select all the text then press Ctrl + I (shortcut for Split Lines). One Disadvantage is that you have to keep your window size steady i.e don't resize it.