Create a mobile App for all operating systems (Android, Apple, Windows

We need a Mobile App written. It must be written to scan a QR code and display a PDF file on Android, Windows and Apple devices. The App will read the QR Code and parse the text string, then pass the string in the form of a URL which will access a PDF record on a database The QR Code will be encrypted but will be passed, as is, to the website as a URL. The receiving 'web page’ will activate a function that will decrypt the code, access a database record, create a PDF file and return it to the App. This will be transparent to the App as it will seem as though the App is merely accessing and displaying a PDF from a website. The App does not, therefore, need to encrypt or decrypt. It must simply pass the text string to the website and display a PDF file on return. We need the app to be compiled for all platforms, we will submit to the App Stores. Our choice is that the App is written in Xamarin but we will consider other options provided that they automatically create Apps for all platforms. If you have Xamarin, we will want a long-term relationship where regular recompilation can be done by you as new devices are launched. We will, nonetheless, require full source code for the App. This is a new area for us so we want to work with someone who can communicate well and where some direct interaction may be needed, on Skype, email and possibly Team Viewer.