How do I search for Activity Attachments?
This might be an answer to a slightly different question, but... in the past, I've used a mix of add-ons to perform a full-text search on activities and their attachments:

https://github.com/civicrm/apachesolr_civiAttachments

When you run a search with "Search => Full Text" and filter on activities, it will include any matches in PDFs/DOCs/etc that are attached to activities.

That module may be a significant undertaking (e.g. it requires an extra server, Solr, and the documentation is pretty rudimentary), but if you really need it, it might be a piece in the puzzle.