Adding text fields and other form elements to create editable PDF forms in Adobe Acrobat Professional XI is a straightforward process. These elements create an interactive document that is ...

Adding Text Notes to a PDF Select the Text Box tool from the Drawing Markups panel. Click, or click and drag, on your document to add a box with a colored border, and type your observations in it.

In Acrobat 5, there's a tool on the toolbar called the "Text Touch-up" tool. Select this, and then you'll be able to select text in the .pdf and replace it.

In other words, Adobe Reader won't let you add text. What you need is a program that's a little more accommodating, PDF-wise. Try Foxit Reader 4.3, which allows you to add text to any PDF.