Change the font size on all your periods 2, 4, or 6 sizes larger to add length to any paper. It's harder to notice the change when the paper is printed out and you'll reach the desired 5, 10, or 20 page assignment quicker.
Guys that won't work because word automatically highlights repeated words so unless your aim is to turn in a hard copy that is pointless. If it is based on word count it will probably be graded digitally so what you can do is add random words at the bottom of the essay and turn the font to a white color.
Sonova...