First you add the comment text to the PDF. Then you need to double click on the added text to enter the edit mode of the text box.
From Acrobat 9, help:
Add text using the Typewriter tool
Use the Typewriter tool to type text anywhere on a PDF page. Organizations sometimes provide PDF versions of their paper forms without interactive form fields. The Typewriter tool provides a simple solution for filling out such forms. The Typewriter tool is like the Text Box tool, but includes a different set of default properties.Note:
Reader users can use the Typewriter tool if the document author enables use of the Typewriter tool for that PDF. To enable use of the Typewriter tool, open the PDF and choose Tools > Typewriter > Enable Typewriter Tool In Adobe Reader.
- Choose Tools > Typewriter > Show Typewriter Toolbar, and then click the Typewriter button.
- Click where you want to type, and then begin typing. Press Enter to add a second line.
- To change the text properties, select the text, and then use any of the following tools in the Typewriter toolbar:
To change the text size, click the Decrease Text Size button or the Increase Text Size button. Or choose a typeface size from the pop-up menu.
To change the line spacing (leading), click the Decrease Line Spacing button or the Increase Line Spacing button.
Choose a color from the Text Color pop-up menu.
Choose a typeface from the typeface pop-up menu.
- To move or resize Typewriter text block, select the Select tool, click a Typewriter text block, and drag the text block or one of its corners.
- To edit the text again, select the Typewriter tool, and then double-click the Typewriter text.
Editing and saving a digitally signed PDF invalidates the signature.