By default, the Advanced Text toolbar contains tools to perform operations on text in the content stream of the PDF.
If the toolbar is currently hidden, go to View > 
Toolbars and select it to show it.
Review Text (SHIFT+ALT+R): Allows for marking text for edits without changing the text in the PDF.
Underline (U): Underlines the selected text.
Squiggly (SHIFT+U): Creates a squiggly line under the selected text.
Strikethrough (D): Strikes out the selected text without removing it.