Page thumbnails are rendered image representations of the page they are generated from.

The SDK API has the flexibility to alter the resolution or size of the thumbnail output and whether to include annotations or not.

To create a basic PDF page thumbnail where the result is a HTMLCanvasElement or HTMLImageElement.

WebViewer(...)
  .then(instance => {
    const { documentViewer } = instance.Core;
    documentViewer.addEventListener('documentLoaded', () => {
      const doc = documentViewer.getDocument();
      const pageNum = 1;
      doc.loadThumbnailAsync(pageNum, (thumbnail) => {
        // thumbnail is a HTMLCanvasElement or HTMLImageElement
        console.log(thumbnail);
      });
    });
  });

To create a high resolution page thumnbails and optionally with annotations where the result is a HTMLCanvasElement or HTMLImageElement.

WebViewer(...)
  .then(instance => {
    const { documentViewer } = instance.Core;
    documentViewer.addEventListener('annotationsLoaded', () => {
      const doc = documentViewer.getDocument();
      const pageNumber = 1;
      const zoom = 2; // render at twice the resolution
      
      doc.loadCanvasAsync(({
        pageNumber,
        zoom,
        drawComplete: async (thumbnail) => {
          const corePageRotation = (doc.getPageRotation(pageNumber) / 90) % 4;
          annotationManager.setAnnotationCanvasTransform(thumbnail.getContext('2d'), zoom, corePageRotation);

          // optionally comment out "drawAnnotations" below to exclude annotations
          await instance.Core.documentViewer.getAnnotationManager().drawAnnotations(pageNumber, thumbnail);
          // thumbnail is a HTMLCanvasElement or HTMLImageElement
          console.log(thumbnail);
        }
      }));
    });
  });