Any media item (including PDFs) is just another type of post. So you need to create a search that will look inside the media folder for all PDFs.
But that won’t help search inside the PDF, though. That’s a bit more complex, requiring the ability to ‘look inside’ the PDF file’s content. And a bit complex to detail here.
But there are plugins that will do the work for you. One is SearchWP , although it is a premium ($$) one. (Which might get me a downvote for recommending plugins, but I am of the opinion that pointing people in the right direction is a good thing.) I have no experience with it, just noticed it via asking the googles.
There are other ‘search enhancers’ that you can find in the plugin directory, or by asking the googles.
You could also use the Google Search process to index (make searchable) your PDFs.