Citations taken from secondary sources
For the entry in the bibliography, cite the compilation as a whole:
Regardless of whether you have consulted archival material in its original form, on microfilm, or in a digital version, your citation must include the elements listed in Chicago 14.120.
In general, you will be safe if you cite the material as if you had seen it in the original form, but you should also include information about the digital collection or microfilm set that you actually used.
Abbreviations. Should you cite certain sources repeatedly, you may wish to develop a system of abbreviations to simplify your footnotes. In this case, a page explaining the abbreviations should follow the table of contents. For example:
DOHC Dulles Oral History Collection
FRUS Foreign Relations of the United States
NYT New York Times