Yes, you can do that. But it will need some coding.

First of all, we need to extract a PDF link or file path from the post. It should be done at indexing stage, inside the 'wpfts_index_post' hook function.

After the link/path extraction, we should read this file and extract the text information from it. It still should be done inside the same hook.

Okay, when we have the text, we put it to one of the index clusters (your preference) and let it to the indexer to make its job as well.

The final code will depend from the way how you have attached the PDF file to the parent post. It can be done using very different ways.

Please give me some more explanation of this and I will post the semi-ready code here.