There are two ways of copying or transporting a supplier invoice (SIN) to a new supplier invoice (SIN):

You can use the "transport all items to another document" function. You find the option in the upper right corner of the supplier invoice (SIN). This function will also bring the attachments over to the new supplier invoice (SIN). If you want to remove the attachment from the new supplier invoice (SIN) you can enter "file manager".  On the attachment you can either press "unlink" or "show links" and then remove the documents from there.

The other way of doing this is using the action menu option: F12 "Copy supplier invoice to new invoice". This will not bring the attachments over. This function is available when the SIN is in ST:4 or higher and LOC=XXX.