Initial thoughts on 4.x upgrade

lib/Pdfjs.4.x.upgrade.MD

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+Let's leave sample app for now. It is easy to rewrite from scratch.
+Let's leave live demo as well. Plan is to rewrite.
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+- Why was ng-packagr added as dev dependency ?
+- Assuming shx works on windows as well. New to this.
+- PDF's are loaded inside an iframe. Does document.addEventListner() gets fired even if we use iframe?
+There are two scenarios. PDF can be embedded in html or opened in a new window.
+- I like the way pdfjs's event bus is used. In the beginning I had to hack pdfjs library pdfjs library to send those browser events. :-)
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+Thought for community - How about starting a discord server for ng2-pdfjs-viewer ?
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