Open the PDF document to modify in the Acrobat Pro application. Click the Forms tab in the right sidebar menu, then click the Edit option. The Add or Edit Form Fields dialog box opens, asking if ...

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 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.