custom-button-toolbar-example.js

const toolbarItems = PSPDFKit.defaultToolbarItems;

let pagerIndex = toolbarItems.findIndex(item => item.type == "pager");
toolbarItems.splice(pagerIndex - 1, 0, {
  type: "custom",
  title: "click me",
  onPress: function() {
    console.log("click");
  }
});

PSPDFKit.load({
  ...defaultConfiguration,
  toolbarItems: toolbarItems
});