Add horizontal line in Word to separate document for easier reading

You can add a horizontal line at the cursor position by clicking the down arrow on the Borders button in the Paragraph group of the Home ribbon, and clicking Horizontal Line in the menu.

But this has nothing to do with being able to see two parts of the document at the same time. For that, go to the View ribbon and click the Split button.

That creates the two views, separated by a line across the window that you can drag up and down. When you don't need it any more, click the same button again, which now says Remove Split.

You can use a paragraph border or you can insert a shape.

In the screen shot below, I have selected a whole paragraph and then I am using the border and shading drop down in the Paragraph group to apply a bottom border to the paragraph.

EDIT: Of course, as Jay has pointed out, adding a "separator" line won't let you scroll two parts of a document independently of each other. For that, you have to split the window.