Not sure about PDF but on on Windows 8 you can create an Image using the Webview.capturePreviewToBlobAsync() function.
It captures everything that's displayed in the Webview and because it's a direct capture anything that can be displayed in IE11 can be captured including MathJax.
I wrote a simple sample app a moment ago and was able to capture the website: to an image. Obviously you could use a URL or insert your own custom HTML.
function printeWebViewToDiskThenDisplay() {
var wc = document.getElementById("webview");
Windows.Storage.ApplicationData.current.localFolder.createFileAsync("webview.png", Windows.Storage.CreationCollisionOption.replaceExisting).then(function (file) {
file.openAsync(Windows.Storage.FileAccessMode.readWrite).then(function (stream) {
var capturePreview = wc.capturePreviewToBlobAsync();
capturePreview.oncomplete = function (completeEvent) {
var inputStream = completeEvent.target.result.msDetachStream();
Windows.Storage.Streams.RandomAccessStream.copyAsync(inputStream, stream).then(function () {
stream.flushAsync().done(function () {
inputStream.close();
stream.close();
var image = new Image();
image.src = URL.createObjectURL(file);
result.innerHTML = "";
result.appendChild(image);
});
});
};
capturePreview.start();
});
});
}
HTML looked something like this:
<button id="btnPrint">Print</button>
<div style="transform:scale(1); transform-origin:0% 0%">
<x-ms-webview id="webview" src="http://cdn.mathjax.org/mathjax/latest/test/sample.html"
style="width: 400px; height: 400px;">
</x-ms-webview>
</div>
<div id="result"></div>
The result was the following:

You can download the rough sample I built here:
https://github.com/thebeebs/ASingleWebviewToImage