Showing Scroll Thumbs

By default, the viewer does never show any scroll bars nor scroll thumbs. You can add scroll thumbs by using PdfViewerParams.viewerOverlayBuilder:

viewerOverlayBuilder: (context, size, handleLinkTap) => [
  // Add vertical scroll thumb on viewer's right side
  PdfViewerScrollThumb(
    controller: controller,
    orientation: ScrollbarOrientation.right,
    thumbSize: const Size(40, 25),
    thumbBuilder:
        (context, thumbSize, pageNumber, controller) =>
            Container(
      color: Colors.black,
      // Show page number on the thumb
      child: Center(
        child: Text(
          pageNumber.toString(),
          style: const TextStyle(color: Colors.white),
        ),
      ),
    ),
  ),
  // Add horizontal scroll thumb on viewer's bottom
  PdfViewerScrollThumb(
    controller: controller,
    orientation: ScrollbarOrientation.bottom,
    thumbSize: const Size(80, 30),
    thumbBuilder:
        (context, thumbSize, pageNumber, controller) =>
            Container(
      color: Colors.red,
    ),
  ),
],

Basically, PdfViewerParams.viewerOverlayBuilder can be used to insert any widgets under viewer's internal Stack.

But if you want to place many visual objects that does not interact with user, you'd better use PdfViewerParams.pagePaintCallback.