Currently, Acumatica Email (send notifications via activities) does not support adding additional attachments on the fly to the built in email features without a cumbersome process of intercepting the activity before it is processed.
This customization allows for selecting additional document attachments to be included with any of the built-in email functions (such as "Email Invoice", etc.)
Must be implemented on a screen by screen basis.
Future: could include generic attachments or collateral such as Terms and Conditions, sales information, etc. - These could be stored at the module level by adding Files to screens such as Sales Order Preferences, etc.