On a desktop computer, hold CTRL and scroll forwards or backwards to zoom in and out. Use the middle mouse button to drag the screen. Use CTRL+M to reset the screen. On a tablet or smartphone, pinch to zoom, and use one finger to drag the screen. Refresh the screen to reset the screen. When zoomed in or out on a touchscreen, you will see a small reset icon at the center bottom:
In Edit Mode the toolbar also offers Zoom out, Zoom in and Fit page buttons and shows the zoom percentage (click it for actual size, or press CTRL+0). CTRL+Shift+M fits the whole page in the window, and holding Space while dragging pans the page without a middle mouse button. Zooming is limited to 10 % to 800 %. See the Page Editor article.