The structure - the only known project in Britain by the American octogenarian - was completed in 2004 as a private initiative to commemorate the millennium.
The site of the scheme was picked to form an axis with the neighbouring 18th-century Oare House.
The clients, Henry and Tessa Keswick, have provided the RIBA archive with models, drawings, correspondence and files.
The pavilion was decorated by John Stefanidis and provides areas for dining, sitting, studying and watching television. The space below is used for the provision of services to the room above.
Bosses of the Portland Place collection are particularly pleased because previously the archive had no original materials relating to Pei.