Problem: You’ve been sent a .pages format document. No, I’d never heard of it either. Apparently it’s a sort of obscure equivalent of the old Windows .wps format, only it’s from word-processing software that ships with Macs. Like the old .wps format used to be, nothing on earth will open it as intended — except its own native production software.
Solution 1: (defunct) It seems there used to be a Windows trick where you’d rename the .pages file as a .zip and there would be a preview.pdf inside that which was usable. That trick doesn’t appear to work any more, at least with the newer versions of whatever Mac software makes .pages files. You may, however, at least get a .JPG image that can show you how the document was meant to look.
Solution 2: (working)
1. Get the free LibreOffice, a version of the popular Open Office suite. The developers of LibreOffice are not afraid to incur the wrath of Apple by enabling .pages import. Install. (LibreOffice has a notoriously long install time, and if you need it a bit quicker then try the portable version).
2. Open a new blank document in the Writer component of Libre Office.
3. Go: top menu bar | Insert | Document and insert your .pages file. Your document loads and appears, but looks like blank pages with a few lines of dots at the top of each page. The formatting has all been lost, but the dots are actually all the words scrunched up together.
4. Go: top menu bar | Edit | Select All | Copy.
5. Open MS Word or similar. Paste in what you just copied to the clipboard. As you’ll see, you’ve lost any fancy formatting there may have been in the .pages file, but at least you’ve got all the plain text and it’s in the right order, it flows and is editable.
Update: Insert | Document is no longer available in the current 2021 version of Writer. Seems to have been removed, presumably at the behest of Apple. Older versions of LibreOffice, inc. summer 2016, are here. Regrettably LibreOffice forces you to uninstall a new version, to install the older version.
Update 2021: There are now several online converters. The only one that worked for me was Zamzar set to convert .pages to a .txt file.