A .pages file is a document from Apple's Pages app. Windows, Android and most software can't open it, which is a problem when someone sends you one. Under the hood a .pages file is a ZIP archive. Documents saved by Pages '09 (and some saved later) include a complete PDF copy of the document, made by Pages itself, for Quick Look previews. This tool looks inside the file in your browser and, when that PDF is there, extracts it exactly as Pages made it, with sharp, selectable text and every page.
Documents saved by Pages 5 and later (2013 onward) usually don't contain that PDF. They hold the document in Apple's own format, which only Pages can lay out, plus a small picture of the first page. In that case the tool says so plainly instead of producing a broken conversion, shows you the preview picture, lets you save it as a one-page PDF or image, and explains how to export the full document as a PDF from Pages on a Mac, an iPhone or iPad, or free at iCloud.com from any computer.
Limits: the full layout of a modern .pages file can't be rebuilt outside Pages, so the preview picture is page 1 only and low resolution. A PDF found inside the file reflects the document as it was when last saved. Password-protected Pages files are encrypted and need to be opened in Pages.