Set your margins and font-size so that the length of each line is around 65 characters.
That’s two and a half alphabets.
A line length of about 65 characters is optimal for reading.
Lines that are much longer than this are harder to read, involving more movement of the eyes and possibly the head, too. This is why magazines and newspapers are typeset in columns.
Unfortunately, the default template (Calibri 11 point with narrow margins) in Word gives a line length of about 90 characters. Change the margins to 30mm on left and right and increase Calibri to 14-point and, there you have it, a readable line length. You could use 13-point Arial or 14-point Times New Roman to get the same result.
If you have a landscape document, unless the font size is very big you’re going to need to put the text into columns (or a table) to keep the line length reasonable, and readable!