I get questions regarding this a lot. I thought that I would clarify what this is, and more importantly is not.
A searchable PDF is defined as a regular PDF with a hidden searchable text layer. So when you have the 100 page PDF open on your desktop, you can find that word or character string within the body of the document.
If you want to be able to search for a word or phrase across a variety of documents in a folder, you are generally talking about something else. This might be indexed documents, which have metadata associated with them, or you might have full text OCR from an Electronic Document Management system, where that hidden layer is part of the database.
When you are considering implementing technology for a “Searchable PDF”, be sure that you are very clear as to what you really mean, and make sure you are able to accomplish your goal. If you have questions, it is better to actually see your solution in action before you buy.