how to extract relavent keyword from documents

You can search for a keyword. A keyword is still just a word, that you can search for inside the file (or inside a long string). Then you can simply just get the content of the document, you're having as a string. And perform string functions over it to find your data in it.

For example, if you're going to find an exact match of a word you can write this code,

string data = "This is your data";
string word = "word";

if(data.IndexOf(word) != -1) {
  // The word is found in the string
}

.. or since IndexOf returns an integer value, you can also get the index of this word in the words of string too. Quite multiple other options too, to seach for a word inside a string of data using C# in ASP.NET.