If your app just downloaded a file and it went into the storage folder, you can use Launcher class to launch an app from the phone which can open the downloaded file for you.
Class StorageFolder will help you get your StorageFile. Considering you already have an object of kind StorageFile, you can use the following line to launch a suitable app automatically from your phone ::
await Launcher.LaunchFileAsync(storageFile);
This will open the app associated with the file type. I hope i got your question clearly and answered in the right direction. Thanks.