When attaching a file as a comment an icon is added to the page letting the user know that a file has been attached. Files attached as a comment are still embedded within the PDF just the same as when attaching a file without a comment. When files are attached directly to a PDF document they are embedded within the PDF file directly so that when the PDF is sent to another user they can still have access to the file.
Note: Attaching a file as a comment does increase the size of the PDF. If size is a concern try using a Link instead that will open the file from another location.
To access the File Comment properties right click (Mac: Ctrl + Click) on the annotation and choose Properties in the mouse menu or click on the edit button on the Mini Toolbar. The File Comment contains the following property settings.
Icon - Sets the icon to be used for the file comment
Transparency - Sets the transparency of the file comment icon
File - Displays the file name associated with this file comment icon