Extract key phrases from a single document
A related keyword to your case can be Single Document Keyword Extraction. A good paper about this is:


  We present a new keyword extraction algorithm that
  applies to a single document without using a corpus.
  Frequent terms are extracted first, then a set of cooccurrence
  between each term and the frequent terms,
  i.e., occurrences in the same sentences, is generated.
  Co-occurrence distribution shows importance of a term
  in the document as follows. If probability distribution of
  co-occurrence between term a and the frequent terms is
  biased to a particular subset of frequent terms, then term
  a is likely to be a keyword. The degree of biases of distribution
  is measured by the $\chi^2$-measure. Our algorithm
  shows comparable performance to tfidf without using a
  corpus.


You can find the paper here.

In sum, this paper gives a rank on keywords based on the defined $\chi^2$-measure.