Hello Yael,
Ritesh is right - SAP offers already different possibilities to add documentation in classification.
You can create std document (CV01n) and link it to a class, a characteristic, a characteristic value.
You can also create a std text.
For example at char. value level:
Then if you send an iDOC, the description and the Std text will be sent. For the document I guess you need to send the document as well.
Hope it helps.
Benjamin