autodesk forge – PixelCompare extensions: compareTwoModels is not a function

I follow the instructions from:

// 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);
  });
});

and get the error: pcExt.compareTwoModels is not a function

It’s meaning this function does not exist in extensions.
Someone can help me How to compare 2 directly pdf files

Thank you so much!