adjust Composable function signature to make source mandatory

[wiryadev]

Current Composable function make all the PDF source as nullable and have default value (null). I think there is at least two issues with that:

1. People can forget to add Pdf source when running the app initially
2. People can accidentally pass multiple source, and get confused why the other source is showing instead what they originally meant to passed

So i propose change to Composable function signature to make it clear that it needs exactly one PDF source to be laoded.

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[deddyrumapea]

I've proposed a different approach to this problem:

#132

I'm curious about your opinion on this.

[github-actions]

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