Where are the long texts of a document stored and how to access the same?

The long texts of a document are stored in a encrypted format in the STXH and STXL tables, where STXH stores the header information of the long text like TDOBJECT, which indicates which text object the long text belongs to, TDID which indicates the Text ID and TDNAME which is the actual name of the long text.

As these texts are stored in a encrypted format, the text cannot be read using a SELECT statement. You will have to use the function READ_TEXT. The easiest way of getting to know the parameter values is to go to a document, open the long text in a full screen mode. For example, when you wan to see the long text for a Purchase order, go to transaction ME23n. Assume, you want to see the parameters for the Header Text. In the first Tab Strip Control, click on the Texts tab and select the Header Text node on the left hand side, which will display the text on the right hand side. Now, double click on the text editor on the right hand side. This will open the text in the full screen mode. In the menu Go To --> Header, you should be able to see the values for all the three parameters we discussed above. We will have to do the same thing for whichever text parameters we want to see.

CALL FUNCTION 'READ_TEXT'
EXPORTING
*   CLIENT                        = SY-MANDT
  ID                            =
  LANGUAGE                      =
  NAME                          =
  OBJECT                        =
*   ARCHIVE_HANDLE                = 0
*   LOCAL_CAT                     = ' '
* IMPORTING
*   HEADER                        =
TABLES
  LINES                         =
* EXCEPTIONS
*   ID                            = 1
*   LANGUAGE                      = 2
*   NAME                          = 3
*   NOT_FOUND                     = 4
*   OBJECT                        = 5
*   REFERENCE_CHECK               = 6
*   WRONG_ACCESS_TO_ARCHIVE       = 7
*   OTHERS                        = 8
        .
IF SY-SUBRC <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.