An archivist is someone who manages and maintains an archive. In the case of PPC archivists, their job is to chronicle, organize, and maintain the records of the PPC's activity, including mission reports. This description leaves itself open to all kinds of Fourth Wall–breaking metaness, given that the Real World writers of PPC stories could sort of be counted as archivists, but for the purposes of in-world continuity, we'll just overlook that, m'kay?
Archivists are technically members of the Department of Personnel, whether they are assigned to another department or a pair of agents or anything else.
There are only four archivists currently known: