Attachment opens are tracked by a small tracking image placed within the attachment. If that image is allowed to load (i.e. the user opens the file) then your console will report the attachment as being opened.
Your user must open the attachment in its native program in order for it to be tracked. (Adobe for PDF attachments, Excel for .XLS attachments, Word for .DOC attachments, and so on.)
For macro attachments, the file is "beaconized" and will "call home" to our servers when the macro is enabled, and the enabled macro will be recorded in your phishing campaign results.