Indeed you will still see the material number that is stored in the transactional data you have archived, however some detailed information about the material is not stored in the transactional movements you archive (in other words: teh complete masterdata is not copied over in the archive file of teh SD invoice) and will therefore be fetched from the online database tables if you retrieve the invoice.
If the material master is archived completely from the system, it could be that you will face retrieval issues if you want to find details about the material used in the invoice (since the masterdata is stored in a different archiving object (MM_MATNR) compared to the SD invoice (SD_VBRK).
Summarized: you can expect some integration issues with retrieval.
There are by the way solutions to compensate that, like PBS.
Regards, Nico Kuijper