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

If you're looking for a quick way to edit or sign a PDF document, there are free and easy-to-use tools for iOS, Windows, and Android devices.

Need to edit a PDF document but don’t want to pay for the more expensive Adobe editors? You can add text to and sign a PDF with just the free Reader program.

Fortunately, Apple has added the ability to electronically sign a PDF document using Preview, a program that comes preinstalled on every Mac running OS X Lion or later.