Indexing pdf and word files using VBA as Microsoft Word Add-in

I would like to build an Microsoft Word Add-in that indexes Word and PDF files.
Then I want to use keywords to search the created index for files with matching content. Much like the Ref-n-write tool.


I am certain that I would have to use VBA for this purpose. I was hoping anyone could shed light on how to proceed.
How can I create a searchable index using VBA?
I could not find any resources on this topic.

Thanks in advance.