What kind of archived documents are you working with? Can they be attributed to a specific author or organization? If so, you could use the author (person or institution) and follow the bibliography reference example from the APA 6th manual. Refer to section 7.10 of the APA 6th manual concerning “Archival Documents and Collections” which provides the following example:
Author, A. A. (Year, Month Day). Title of material [Description of material]. Name of Collection (Call number, Box number,
File name or number, etc.). Name of Repository, Location.
If you’re able to pinpoint a specific author or organization then the in-text citation could follow the regular Author-Date format of Endnote’s APA 6th output style file. However, like the Book or Journal Article reference types, you might consider creating a reference type specifically for Archival Documents. This allows categorizing of the Endnote records and creating a bibliography template in Endnote’s APA 6th output style that follows the format of section 7.10 of the APA manual.