PixelCompare extension

Previously you needed a custom version of the PixelCompare extension in order to use it in your app - see Compare two 2D documents using Forge Viewer

Though it does not seem to be documented on the Forge website, we did provide information on the improvements it has been getting - see Viewer Release Notes: v 7.4

It's really simple to use.
You just have to load two sheets into the model (e.g. two single page PDF files) and call compareTwoModels() for those models.

I'm using this code to load my two sample PDF files from my computer - see result in picture on top

function launchViewer() {
  var options = {
    env: 'Local'
  };

  Autodesk.Viewing.Initializer(options, () => {
    viewer = new Autodesk.Viewing.GuiViewer3D(
      document.getElementById('forgeViewer'), {}
    );
    
    viewer.start();

    // Load 2 sheets
    viewer.loadModel('scissors1.pdf', {}, (model1) => {
      viewer.loadModel('scissors2.pdf', {}, async (model2) => {
        // Compare them 
        const pcExt = await viewer.loadExtension('Autodesk.Viewing.PixelCompare');
        pcExt.compareTwoModels(model1, model2);
      });
    });
  });
}

The above code could also be used to load a PDF directly (without it being translated to SVF, by relying on the PdfLoader) from any storage location including OSS (inc BIM 360 Docs, Fusion Team) by providing the URL to the file, e.g. 
Note: if the file is on OSS then you'll also have to provide the getAccessToken option as well for the Initializer function

viewer.loadModel("https://developer.api.autodesk.com/oss/v2/buckets/my_bucket/objects/my.pdf", {}, async (model1) => {
// etc

Here is how you can find a file's storage location if it's stored in BIM 360 Docs or Fusion Team: Download a File 

If you were trying to compare sheets in a Revit document (or a PDF that has been translated to SVF), then better to use Autodesk.Viewing.Document.load() and viewer.loadDocumentNode() instead:

function launchViewer() {
  var options = {
    env: "AutodeskProduction",
    getAccessToken: getForgeToken
  };

  Autodesk.Viewing.Initializer(options, () => {
    viewer = new Autodesk.Viewing.GuiViewer3D(
      document.getElementById('forgeViewer'), {}
    );
    
    viewer.start();

    Autodesk.Viewing.Document.load('urn:dXJuOm...', doc => {
      var viewables = doc.getRoot().search(
        {
          type: "geometry",
          role: "2d",
        },
        true
      );
      
      // Load 2 sheets
      viewer.loadDocumentNode(doc, viewables[0]).then((model1) => {
        viewer.loadDocumentNode(doc, viewables[1], { keepCurrentModels: true }).then(async (model2) => {
          // Compare them
          var pcExt = await viewer.loadExtension("Autodesk.Viewing.PixelCompare");
          pcExt.compareTwoModels(model1, model2);
        })
      });
    });
  });
}