Rudolf Engel obtained a doctorate in philosophy from the University of Vienna in 1906. He worked as a secondary school teacher but was mainly involved in publishing and selling books and theatre items. In 1923, he founded EOS-Verlag, specializing in erotica and the publication of French and Latin works and their German translations. From March 1935 to September 1937 his wife Henriette Engel, née Wostry (1890–1965), owned Theaterbuchhandlung Wallishauser, founded in 1789, in Vienna's 1st district, whereas he ran Cottage-Buchhandlung in the 18th district for several years. Engel gave up his bookselling licence in October 1936. His company was deleted from the registry of companies in February 1941 following an application made the year before. From September 1938 Rudolf and Henriette Engel lived in Kirchberg am Wechsel in Lower Austria. It is unknown whether Engel worked there. He made contradictory statements regarding his NSDAP membership. In 1938 he claimed to be a member, but in a questionnaire in 1943 for admission to the Reichsschrifttumskammer (Reich Literature Chamber) he denied membership of the NSDAP or any of its bodies. After returning to Vienna in October 1946, he advertised in the magazine Das Antiquariat as Buchhandlung Kirchberg am Wechsel. The register of companies gives no further information about other business activities, but until his death in 1955 he sold numerous books and theatre items to the Austrian National Library, the University of Vienna library, the Austrian Theatre Museum and the Theatre Research Collection of the University of Cologne.

In 2009, the Art Restitution Advisory Board recommended the restitution of the items of the major Schönbrunnensia collection of Ernst Moriz Kronfeld (1865–1943), which the National Library had purchased from Engel in 1942. It is not known how they came into Engel's possession.