File format is invalid

What is the document name including extension? What version of Pages (Pages : About Pages), and version of operating system are you on ( menu : About this Mac)?

Any other Pages dialog messages about this document?

Did you rename some file thinking changing the extension to .pages, .doc, .docx, .rtf, or .rtfd would convert it? Was the document double-clicked on Google Drive or on Microsoft Windows?

Open any Pages document on your iPhone. Then, press ⋯⃝ and then click Help : What's New in Pages. This should tell you the Pages version number that is installed on the iPhone.