I doubt you'll find a lot of tutorials about doing this, but it's not that hard if you just break it down into simple steps.

Create an XML file that contains nodes that consist of file names and keywords, where the keywords describe the documents that you want to make searchable. Something as simple as this could work.

Code:
<files>
  <file>
    <filename>whatever.doc</filename>
    <keywords>lorem ipsum dolor sit...</keywords>
  </file>
  <file>
    <filename>document.pdf</filename>
    <keywords>keywords extracted from the PDF or just entered manually</keywords>
  </file>
</files>
Let the user type in some keywords.

Load the XML file and search for the best match between the keywords the user typed and the keywords in the XML.

Present the user with a list of files that matched their keywords and let them open that file.

The opening of the file can be done using one of the techniques described here over the years, whether that's a third party swf2exe tool or using something like my free proxy tool.