Donwnload File - Dynamic Filename

Below is the code snippet

The type should be String as Character(C) data type will generate error 

DATA: l_filename TYPE STRING. 

CONCATENATE 'c:\MPF_REPORT' sy-datum sy-timlo sy-uname '.pdf' INTO l_filename.
CONDENSE l_filename.

CALL FUNCTION 'GUI_DOWNLOAD'
  EXPORTING
*   BIN_FILESIZE                    =
    FILENAME                        = l_filename "'c:\MPF_REPORT20110921063337.pdf' "'c:\test_pdf_sf.pdf'
    FILETYPE                        = 'BIN'
*   APPEND                          = ' '
*   WRITE_FIELD_SEPARATOR           = ' '
*   HEADER                          = '00'
*   TRUNC_TRAILING_BLANKS           = ' '
*   WRITE_LF                        = 'X'
*   COL_SELECT                      = ' '
*   COL_SELECT_MASK                 = ' '
*   DAT_MODE                        = ' '
*   CONFIRM_OVERWRITE               = ' '
*   NO_AUTH_CHECK                   = ' '
*   CODEPAGE                        = ' '
*   IGNORE_CERR                     = ABAP_TRUE
*   REPLACEMENT                     = '#'
*   WRITE_BOM                       = ' '
*   TRUNC_TRAILING_BLANKS_EOL       = 'X'
*   WK1_N_FORMAT                    = ' '
*   WK1_N_SIZE                      = ' '
*   WK1_T_FORMAT                    = ' '
*   WK1_T_SIZE                      = ' '
*   WRITE_LF_AFTER_LAST_LINE        = ABAP_TRUE
*   SHOW_TRANSFER_STATUS            = ABAP_TRUE
* IMPORTING
*   FILELENGTH                      =
  TABLES
    DATA_TAB                        =  i_pdf
*   FIELDNAMES                      =
* EXCEPTIONS
*   FILE_WRITE_ERROR                = 1
*   NO_BATCH                        = 2
*   GUI_REFUSE_FILETRANSFER         = 3
*   INVALID_TYPE                    = 4
*   NO_AUTHORITY                    = 5
*   UNKNOWN_ERROR                   = 6
*   HEADER_NOT_ALLOWED              = 7
*   SEPARATOR_NOT_ALLOWED           = 8
*   FILESIZE_NOT_ALLOWED            = 9
*   HEADER_TOO_LONG                 = 10
*   DP_ERROR_CREATE                 = 11
*   DP_ERROR_SEND                   = 12
*   DP_ERROR_WRITE                  = 13
*   UNKNOWN_DP_ERROR                = 14
*   ACCESS_DENIED                   = 15
*   DP_OUT_OF_MEMORY                = 16
*   DISK_FULL                       = 17
*   DP_TIMEOUT                      = 18
*   FILE_NOT_FOUND                  = 19
*   DATAPROVIDER_EXCEPTION          = 20
*   CONTROL_FLUSH_ERROR             = 21
*   OTHERS                          = 22
          .
IF SY-SUBRC <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.