For this exception occurs due to missing native code for all the platforms, refer below the example android we should write the following code on the native code.
https://github.com/syncfusion/flutter-examples/blob/master/android/app/src/main/java/com/syncfusion/flutter_examples/MainActivity.java#L27
Please refer the following package to your project:
https://pub.dev/packages/open_file
Please follow the below links to understand the usage of this package in your project:
https://www.syncfusion.com/kb/11961/how-to-read-pdf-file-in-syncfusion-flutter-pdf-library
Follow the above steps and let us know the result. If still, you are facing the issue. We request to share the sample and the more details about the exception like issue screenshots/ documentation/ video demos to understand the issue more clearer and provide you the accurate details