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You’re right.

For a long time, many users were reporting errors when trying to use the plugin with Internet Explorer that I could not reproduce myself. To get around these problems, I just defaulted to serving the PDF itself in an iFrame to Internet Explorer.

You can remove the conditional comments from the plugins source if you’d like to remove this behavior.

All the trouble with Internet Explorer happened a few years ago now. I’m not sure if Internet Explorer has fixed whatever was causing these issues but, as I said, I was never able to reproduce the problem myself, so I cannot say for sure if IE has changed at all.