By default, the Text toolbar contains tools used to create text markups and other related tools.
If the toolbar is currently hidden, go to View > 
Toolbars and select it to show it.
Text Box (T): Places a text box markup.
Typewriter (W): Enables freely typing text on a PDF.
Note (N): Inserts a sticky note.
Flag (F): Inserts a linked flag on the currently active page.
Pen (P): Adds a free-drawn markup.
Highlight (H): Adds a semi-transparent line simulating a highlighter.
Eraser (SHIFT+E): Removes pen and highlighter markups.