To annotate PDF documents, you will tap on the "annotate" icon in the top right corner. A tool bar will show on the left side
To type, select the second icon on the tool bar, the text box will appear and you can add the text.
There are many options for annotating like a pen tool, highlighter, adding shapes and images.
All of the tools with multiple options can be opened up by holding down on the icon.
You may also reference the attached video to see the annotations tool in action!