Yes, you can restrict the files you want using the Filters field, as described in this documentation.

That said, you are most likely going to be very disappointed if you try to search PDFs or Word documents with Notepad++, because Notepad++ is a text editor, not a word processor.

If you want to search all kinds of documents (not just text documents), I recommend dnGrep, a free and open-source file searching tool. That said, further discussion of dnGrep would be off-topic for this forum.