Hi Ravi,
If you want the text at mail body,You can use this class while creating the document.
lcl_document = cl_document_bcs=>create_document(
i_type = lc_type
i_text = lt_text
i_length = lc_length
i_subject = ls_subject ).
In lt_text you can give the required text.
If you want at mail subject level,you can use
CALL METHOD lcl_document->add_attachment
EXPORTING
i_attachment_type = lc_pdf
i_attachment_subject = ls_mail_sub
i_attachment_size = lv_pdf_size
i_att_content_hex = lt_pdf_content.
In ls_mail_sub you can give teh required text in addition to the other text.
Thanks & Regards,
Vishnu