How to add a document(s) to a Page Library
Once a Page Library has been created, it's ready for you to add documents into it.
Page Libraries must only contain pages in the same format (e.g. Book page or Presentation page, not a mix)
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Select 'Manage' for the document from your homepage.
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Select 'Manage Version', followed by 'Version Settings'.
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Choose which Page Library you would like to add it to from the 'Part of Page Library' drop-down list.
Documents that are added to a Page Library cannot be published as they are being used by other documents. This also means you can edit the documents at any time too, which will update the library!

Read the Page Libraries Guide - https://view.pagetiger.com/TigerLabs/page-libraries