Via IMPORTING-parameters IV_VIEWER_CALLBACK_PROG and IV_VIEWER_CALLBACK_FORM, You able to specify a CALLBACK-subroutine, that should be called at some events of the Viewer: 

• BEFORE_OLE_INITIALIZATION – is being raised in the Dynpro-PBO, before creating OLE-objects and showing the form via MS-Excel application. Here you can use third-party tools to bring some changes to the document, which available as changing-parameter cv_rawdata (HEX-sequence) .

• AFTER_OLE_INITIALIZATION – is being raised in the Dynpro-PBO, before creating OLE-objects and showing the form via MS-Excel application.  Here you can use OLE or DOI-technology for some manipulations with the form. For retrieving the actual form DOI-object, you should use subroutine VIEWER_GET_DOI_OBJECT. 

• CONTROLS_INIT – is being raised in the Dynpro-PBO, just once – at controls initialization (containers and toolbar). At the moment You able to add customer buttons to the toolbar.

• FUNCTION_CODE – is being raised on any user-defined button click.

• REDRAW – being raised at jump to a next workbook in the bundle.

Example of CALLBACK-subroutine using:

CALL FUNCTION 'ZXLWB_CALLFORM'

  EXPORTING

    iv_formname             = 'SHIPPING_LABELS'

    iv_context_ref          = gt_context[]

    iv_viewer_callback_prog = sy-cprog

    iv_viewer_callback_form = 'CALLBACK_FORM'

  EXCEPTIONS

    OTHERS                  = 2.

IF sy-subrc NE 0 .

  MESSAGE ID sy-msgid TYPE sy-msgty NUMBER sy-msgno

          WITH sy-msgv1 sy-msgv2 sy-msgv3 sy-msgv4 .

ENDIF .

*&---------------------------------------------------------------------*

*&      Form  CALLBACK_FORM

*&---------------------------------------------------------------------*

FORM callback_form

     USING pv_event     TYPE char50

  CHANGING cv_fcode     TYPE ui_func

           cr_toolbar   TYPE REF TO cl_gui_toolbar

           cv_rawdata   TYPE xstring . " -->>  excel document as hex-string 

  CASE pv_event .

*=======================================================================

    WHEN 'BEFORE_OLE_INITIALIZATION' . "  here you can bring some changes to the CV_RAWDATA

*                                         (excel document as HEX-string)

*=======================================================================

*=======================================================================

    WHEN 'AFTER_OLE_INITIALIZATION' .  "  here you can use OLE or DOI

*                                         to post-processing the excel document

*=======================================================================

*     get DOI object

      DATA:

        lo_container_control    TYPE REF TO i_oi_container_control ,

        lo_document_proxy       TYPE REF TO i_oi_document_proxy ,

        lo_spreadsheet          TYPE REF TO i_oi_spreadsheet .

      PERFORM viewer_get_doi_object

      IN PROGRAM saplzxlwb

        CHANGING lo_container_control

                 lo_document_proxy

                 lo_spreadsheet .

      CHECK lo_spreadsheet IS BOUND .

*     CASE 1: print out an actual worksheet without print-dialog

      lo_spreadsheet->print( name = 'Sheet1' ) .

*     CASE 2: show print-dialog via DOI

      lo_document_proxy->print_document(

        EXPORTING prompt_user = abap_on

                  no_flush    = abap_off ) .

*     CASE 3: show print-dialog via OLE

      DATA:

        ls_handle              TYPE cntl_handle ,

        ls_application         TYPE ole2_object ,

        ls_activeworkbook      TYPE ole2_object ,

        ls_dialogs             TYPE ole2_object ,

        ls_item                TYPE ole2_object .

      lo_document_proxy->get_document_handle( IMPORTING handle = ls_handle ) .

      CALL METHOD OF:

         ls_handle-obj    'Application' = ls_application,

         ls_application   'Dialogs'     = ls_dialogs,

         ls_dialogs       'Item'        = ls_item EXPORTING #1 = '8',

         ls_item          'Show'.

*     CASE 4: save with protecting worksheet

      GET PROPERTY OF ls_application 'ActiveWorkbook' = ls_activeworkbook .

      CALL METHOD OF

          ls_activeworkbook 'SaveAs'

        EXPORTING

          #1                = 'C:\protected.xlsx'

          #2                = 51    " -->> FileFormat:=xlOpenXMLWorkbook

          #3                = 'password'.

      FREE OBJECT ls_item .           CLEAR ls_item .

      FREE OBJECT ls_dialogs .        CLEAR ls_dialogs .

      FREE OBJECT ls_activeworkbook . CLEAR ls_activeworkbook .

      FREE OBJECT ls_application .    CLEAR ls_application .

*=======================================================================

    WHEN 'CONTROLS_INIT' .    " here you can add custom buttons to the toolbar

*=======================================================================

      TYPE-POOLS: cntb , icon .

      cr_toolbar->add_button( fcode     = 'NEW_BUTTON'

                              text      = 'Show my message'

                              icon      = icon_message_warning_small

                              butn_type = cntb_btype_button ) .

*=======================================================================

    WHEN 'FUNCTION_CODE' .    " here you can process function codes

*=======================================================================

      CASE cv_fcode .

        WHEN 'NEW_BUTTON' .

          MESSAGE 'My message' TYPE 'I' .

        WHEN OTHERS .

      ENDCASE .

*=======================================================================

    WHEN 'REDRAW' .           " here you can process a jump

*                               to a next workbook in the bundle.

*=======================================================================

  ENDCASE .

ENDFORM .                    "CALLBACK_FORM

Result (a customer-defined button in the application toolbar):