01Why can Windows not open a .pages file?
Because it is not a document in the usual sense: it is a package, a ZIP archive holding Apple proprietary content files, and Windows has no application that reads them. Since 2013 the content sits in Index.zip as IWA, an undocumented binary dialect of protocol buffers that changes between iWork releases, which is why no third-party editor supports it properly and why no browser tool can genuinely convert one. Rename the file to .zip and you can look inside yourself: you will find Index.zip, a Data folder, a Metadata folder and a few JPEGs.
02Why do I only get the first page?
Because that is all the file contains. Up to iWork '09 Apple embedded QuickLook/Preview.pdf, a real PDF of the entire document, and when your file is that old this tool hands you exactly that. The 2013 rewrite removed it and replaced it with three JPEGs (preview.jpg, preview-web.jpg and preview-micro.jpg) that exist to draw the icon in Finder, so they cover the first page only. No tool can extract pages that were never stored, and any site promising a full conversion of a modern .pages is either rendering it server-side with a real copy of Pages or overpromising.
03How do I get the complete document from a modern iWork file?
You need something that can actually render iWork, and there are three free routes. Upload it to iCloud.com and open it in the web version of Pages, Numbers or Keynote, then export to PDF: this works from any browser with a free Apple ID. Ask the sender to export a PDF, which takes them one menu. Or open it on any Mac, iPhone or iPad, where the apps are free. The trade-off is the one this site usually cares about: iCloud means uploading your document to Apple, while this page does not.
05What is actually inside a .pages, .numbers or .key file?
A ZIP package holding the document as compressed binary archives in Apple internal IWA format, plus, when the save included it, a ready made preview: a PDF or an image of the document. There is no readable text or markup inside, which is why Word, LibreOffice and search tools see nothing. The only universally readable thing the package carries is that preview, and it is exactly what this tool extracts.
06How do I open a .pages file in Word?
Word cannot read the format, in any version. There are three honest routes: extract the preview PDF here when you only need to read or print the document; open the file at iCloud.com with a free Apple ID and export it to .docx when you need to edit it; or ask the sender to export it as Word or PDF, which takes ten seconds in Pages.