You can add a list of file id's or attachment id's to the variable object_id_list to give a specific set of documents a user can choose to send in the pop-up. You also need to set whether it's a list of files or notes & attachments using object_id_list_type. It's up to you to generate the list of id's.
Within window.SR_SETTINGS:
object_id_list: ['234rwe4234rweSEF', '234rwe4234rweSEF', '234rwe4234rweSEF', '234rwe4234rweSEF', '234rwe4234rweSEF'],
object_id_list_type: 'Files' , // Or use Attachments