Various style guides have recommendations for citing indirect sources. However it's important to keep in mind that your works cited page is a list of sources you personally have used during your research. Since that's the case, it's recommended (generally) that you don't use indirect sources, but that if you do chose to use indirect sources, you let the audience know where you're gathering your information.
You shouldn't cite the main paper, as you are not quoting material, nor gathering information directly from the source. You should quote the source where you directly culled your information from, as it provides the most accurate information to your audience.
Here are how various style guides recommend dealing with indirect sources:
Chicago recommends the following when quoting an indirect source:
Because authors are generally expected to be intimately familiar with the sources they are citing, Chicago discourages the use of a source that was cited within another (secondary) source. In the case that an original source is utterly unavailable, however, Chicago recommends the use of "quoted in" for the note:
- Ian Hacking, The Social Construction of What? (Cambridge, MA: Harvard University Press, 1999), 103, quoted in Manuel DeLanda, A New Philosophy of Society (New York: Continuum, 2006), 2.
If you don't have access, at all, to the other source, you should, according to CMS, quote the source you're referencing. In this case, you should quote the slideshow, to let outside readers know that you have not accessed the primary document and are relying on someone else's information.
MLA style asks for much of the same:
Sometimes you may have to use an indirect source. An indirect source is a source cited in another source. For such indirect quotations, use "qtd. in" to indicate the source you actually consulted. For example:
Ravitch argues that high schools are pressured to act as "social service centers, and they don't do that well" (qtd. in Weisman 259).
Note that, in most cases, a responsible researcher will attempt to find the original source, rather than citing an indirect source.
For good measure, here's what APA asks for:
If you use a source that was cited in another source, name the original source in your signal phrase. List the secondary source in your reference list and include the secondary source in the parentheses.
Johnson argued that...(as cited in Smith, 2003, p. 102).
Note: When citing material in parentheses, set off the citation with a comma, as above. Also, try to locate the original material and cite the original source.
In all cases, the writers are citing where they found the information, not where that information originally appeared.