use 'convert_otf', but the output table of lines is empty, why?

Hello experts,

I am trying to use the function 'convert_otf' to convert smartforms into pdf. But the output table of lines is empty´.

*   Convert OTF into PDF
CALL FUNCTION 'CONVERT_OTF'
  EXPORTING
    format                = 'PDF'
*    max_linewidth         = 132
  IMPORTING
    bin_filesize          = lv_bytes
    bin_file              = bin_file
  TABLES
    otf                   = gt_otfdata
    lines                 = gt_pdfdata
  EXCEPTIONS
    err_max_linewidth     = 1
    err_format            = 2
    err_conv_not_possible = 3
    OTHERS                = 4.

Before calling this function the parameter value are like following:

LV_BYTES: 0
BIN_FILE: 
GT_OTFDATA: Standard Table[15x2(144)
GT_PDFDATA: Standard Table[0x2(268)]

After this function is called, the parameter value are like this:

LV_BYTES: 1774
BIN_FILE:  255044462D312E330D0A2...
GT_OTFDATA: Standard Table[15x2(144)
GT_PDFDATA: Standard Table[0x2(268)]

It looks that there is output for table lines. For download this pdf I have to get a table of pdf data.

What could be the reason for this?

Thank you.

Rujing