This article show three methods that can be used to add audio comments to PDF documents in Adobe Acrobat free and Pro version.

How to Add Comments to a PDF With Acrobat Reader. PDF files enable you to share and comment on content created in a wide range of applications. Adobe Reader, formerly called Acrobat Reader, offers ...

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.