Here's a link to the Index Search Overview topic in the Windchill Help Center:
Take a look at that as well as the other Topics under the "Windchill Search Administration" topic for the documentation on full text search.
You'll need to install the Windchill Index Search module to use full text search. Then, you'll need to run the Bulk Indexing Tool to actually index your indexable content files. And yes, both PDF and Excel files are indexable.