To add stamps to your document, tap on the icon on the toolbar above the viewer and choose a stamp style.
Then put the stamp on the document where you want it to be. Before the stamp is applied, it has a transparent look. Once it's applied on the document, it will look solid.
You can tap on the stamp and adjust its size by dragging the corners. To delete a stamp, right click (or long press on a touch screen) and select "Clear Markup."