If you inserted page numbers by using the Page Numbers command on the Insert menu, make sure to select the frame (frame: A container that you can resize and position anywhere on the page. To position text or graphics that contain comments, footnotes, endnotes, or certain fields, you must use a frame instead of a text box.) around the page number.
How?
Click the page number to make its frame appear.
Move the pointer over the frame's border until the pointer becomes a four-headed arrow, and then click to see the frame's sizing handles (sizing handle: One of the small circles or squares that appears at the corners and sides of a selected object. You drag these handles to change the size of the object.).
If the handles are not visible, you might have clicked the contents of the frame, not the frame itself.
Notes