Get selected text from Chrome PDF viewer, `getSelection.toString()` always returns empty string

Question

I have a simple HTML page with a button, and an iframe displaying a PDF file. When the button is clicked, I need to get the selected text in the PDF if any, and store it in a variable.

So far I have tried options from this question, among others. However it seems that getSelection.toString() is always returning an empty string.

By the way I am using Chrome with its default PDF viewer. Is there a way to achieve the above?

My code so far:

$(document).ready(function() {

  $("#btn-get-selection").on("click", function(e) {
    var iframe = document.getElementById('iframe');
    var idoc = iframe.contentDocument || iframe.contentWindow.document;
    var text = idoc.getSelection().toString();

    //always getting empty string
    console.log(text);
  })
});

<button id="btn-get-selection" class="btn btn-primary">Get Selection</button>

<iframe id="iframe" src="sample.pdf" width="100%" height="500" frameborder="0"></iframe>

Current output:

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Edit:

Using PDFjs instead of the Chrome default pdf viewer, the code works. A quick way to get PDFjs running can be found here.