When you tap the magnifier icon, or just hover the mouse over it, a zoom slider. pop-up is shown that lets you interactively adjust the zoom level from 30% to 1000%. 

Pressing the + icon will also smoothly increase the zoom level while pressed (and will go beyond 1000%).

Toggle between the previous and current zoom levels by tapping the button at the far left of the slider.

Tap the [<>] icon to fit the document to the width of the workspace.  

Zoom to 100% by tapping the vertical line on the slider.

A direct way to zoom is to use the mouse scroll wheel in conjunction with the Shift or Ctrl key, which allows fast zoom all the way to 25,000%. 

This also has the advantage that it zooms into the mouse pointer location. 

Drag the magnifier icon up the left edge of the screen to zoom in.

Drag right along the bottom edge to zoom out. This zooms around the center of the screen, but you can set a zoom focus point by clicking on the magnifier and clicking on the point you want to zoom into. After this dragging the slider will zoom around that point on the page.

Click or tap the magnifier icon and marquee drag around the area you want to zoom into.

Z will shortcut straight to marquee zoom.

Ctrl+0 (zero) key will zoom to 100% view.

Ctrl with (+) plus and (-) key to zoom in /out one step.

On a touch based device you can use pinch-zoom gesture.

Alt+F will fit the document to the width of the workspace.