Currently, you cannot add page numbers directly when previewing documents or invoices in your browser. However, page numbers can be included when downloading PDFs using an Organization setting.
Steps to Enable Page Numbers:
Go to Settings on the left-hand sidebar.
Under Organization, click General Settings.
Under Documents, click Edit.
Toggle Show Page Numbers in Bottom Margin of Document and Invoice PDFs.
Click Save.
Once enabled, PDFs of Documents or Invoices will display the option to show page numbers in the bottom margin.