How to open files in default program?

Now you can use the Launcher class by adding a reference to Xamarin.Essentials in your class.

For opening a file by another application use:

await Launcher.OpenAsync(new OpenFileRequest
{
    File = new ReadOnlyFile(file)
});

Where file is the File or FullPath.

You can find more here: https://learn.microsoft.com/en-us/xamarin/essentials/launcher?tabs=android

It depends on what type of documents you are taking about. In Xamarin.Forms, you can use

Device.OpenUri(new Uri("filepath or http url"));

If there is an application that can handle the extension, then it will prompt you on which app you wish to use.

If you wanted to view them internally, inside the app, without launching a 3rd party app, then we really need to know what type of documents you are opening. If it's a PDF, then you can look at Display Local PDF File In WebView.

Should it be another type of document, you will need to see if there is a different