One problem I have encountered while doing research is that I find it difficult to find papers that were published decades ago. In particular, I am interested in
"Extension of a theorem of Moon and Moser on complete subgraphs" - de Caen, 1983
and the proof of the result that the Turan density upper bound for complete r-graphs is $1 - \frac{1}{r}$. I believe this result was also proved by Sidorenko (and I vaguely remember seeing somewhere that two Japanese mathematicians had also proven this result, all of them around the early 1980s).
Over time, many results in Turan density upper bounds have referenced this but I cannot find it online. I should mention that I am a college student (in case it helps any with finding this paper).
Some papers you may find of interest regarding upper and lower bounds of Turán Densities of complete k-uniform hypergraphs:
Sidorenko A. (1995), "What we know and what we do not know about Turán numbers", Graphs and Combinatorics, 11 (2): 179–199, doi:10.1007/BF01929486
Sidorenko A. (1997), Upper Bounds for Turán Numbers, journal of combinatorial theory, Series A 77, 134-147
Some more recent results:
An excellent and comprehensive survey on the subject:
Keevash P. (2011), Hypergraph Turán problems, Surveys in combinatorics 392: 83-140.
New upper bound on the Turan density of the 4-uniform K_5:
Markström K. (2009), Extremal hypergraphs and bounds for the Turan density of the 4-uniform K-5, Discrete Mathematics, Vol. 309, no 16, p. 5231-5234, DOI: 10.1016/J.Disc.2009.03.035
New lower bound on the Turan density of the 4-uniform K_6:
Montecalvo F. (2014), Asymptotic Bounds for General Covering Designs, J. Combin. Designs 23: 18–44, DOI 10.1002/jcd.21401