I presume that you start with a list of such keywords?
You can create a script in the Javascript programming language that can be run when a file is placed in a folder. Check the Alfresco wiki for more information about creating such scripts.
You can have the script invoke the full text search services to search the content of the file. You would constrain the search by including the full path to the containing folder, so that only files in the local folder were to be searched. Then, match the search results against the current file. Each keyword would be done as a separate search.
Alternatively, if you were working with text files, you could look inside the content of each file using a pattern to pick off the list of keywords.
The advantage to the full text search services is that they understand all of the document formats that Alfresco supports, and can be reconfigured to support international character sets, and rules of languages other than English.