Allow to use alternative source for PDF.js loading

[MacDeveloper1]

In noticed that the predefined CDN for PDF printing (https://unpkg.com/pdfjs-dist) is often returns the 502 error. So that inspire me to add functionality when user can change the CDN or even use local copy of pdf.js.

To change the CDN base URL user have to created a JavaScript variable and initialize it with the directory which contains files pdf.min.js and pdf.worker.min.js.

<script>
  var dartPdfJsBaseUrl = "https://cdnjs.cloudflare.com/ajax/libs/pdf.js/3.2.146/";
</script>

To use local storage for PDF scripts, save the required files into the new directory insside the root directory of site and initialize the variable.

<script>
  var dartPdfJsBaseUrl = "assets/js/pdf/";
</script>

[DavBfr]

Merged, thanks.