Pages is Apple's word processor (the macOS/iOS counterpart to Word). Files are zip-packaged with proprietary iWork XML inside, not OOXML, not openable outside Apple's ecosystem without conversion. macOS/iOS Quick Look shows a thumbnail; reading the actual content on Windows/Linux requires either iCloud Pages (web) or converting to PDF/DOCX first.
How to open
Pages on macOS/iOS (free with any Apple device since 2013). iCloud.com Pages on any browser. Convert to PDF or DOCX for Windows/Linux compatibility.