From the English Wikipedia page 
Juan Menéndez Pidal (1861–1915) was a Spanish archivist, jurisconsult, historian, and poet, brother of Luis and Ramón Menéndez Pidal. He was long a director of the Archivo Histórico Nacional at Madrid, and a director of the Revista de Archivos, Bibliotecas, y Museos.
Born at Madrid, he studied law until he obtained his title, and then devoted himself to journalism. His Dios y César, an ecclesiastical and legal study of the relation of church and state, attracted much attention. In 1914 he was elected a member of the Spanish Royal Academy of the Language.
There is slightly more information, and a photograph, on the Spanish Wikipedia page .
See also List of archivists