After tapping an annotation command (such as Pencil, Highlighter, or Underline), the Annotation Ribbon appears in the upper-right. The button there marked “Scroll” allows you scroll through the document to find the desired location for an annotation. The “Markup” button, in the same position on the Annotation Ribbon, allows you to toggle back to Markup mode. Textual markups, such as the Highlighter, also allow two-finger scrolling through the document, even while marking it up.

Also, double-tapping the screen will toggle between Scroll and Markup modes. However, this toggling feature cannot be used while making new Pencil annotations (i.e., the app remains in Markup mode despite any double-tapping).