Adapt an import in a CoffeeScript file to the ES module conversion

This module has no tests :(

src/annotator/plugin/pdf.coffee

@@ -9,7 +9,7 @@ module.exports = class PDF extends Plugin
 
   pluginInit: ->
     @annotator.anchoring = require('../anchoring/pdf')
-    PDFMetadata = require('./pdf-metadata')
+    { default: PDFMetadata } = require('./pdf-metadata')
 
     @pdfViewer = PDFViewerApplication.pdfViewer
     @pdfViewer.viewer.classList.add('has-transparent-text-layer')