If you add comments to a PDF that isn’t part of a managed review, you may need to export your comments to send them to someone, or you may need to import comments you receive. (PDFs in a managed review workflow include special options that let you send or publish your comments, rather than export them.)
When you export comments, you create a Form Data Format (FDF) file that contains only comments. Consequently, FDF files are usually smaller than PDFs. You or another reviewer can then import the comments from the FDF file into the original PDF.
Export comments to a data file
Export selected comments