Sam here replying to your post. This can be done relatively easy. Not sure what the app store has to offer but investment wise better to build the functionality into a private app. Why? Because down the road you could add more functionality if you wish. There are many PDF reader/writer libraries for several popular programming languages. The only challenge is getting the details and testing it out.
The cost should be proportional to the raw material and labor. We can host your app on Amazon Web Services for a small monthly fee plus the development time to get the functionality you're looking for. Of course we want to know what your PDF will look like and any additional functionality that should exist prior or after generating and/or emailing the PDF to customer. If you email us at firstname.lastname@example.org I can speak with you in depth about your requirements and finally email you a PDF quote that you can think over.