It it is a Pages '09 package format file — you can use the Finder to right-click on this document, choose Show Package Contents, and then navigate down into its QuickLook directory. There, the Preview.pdf document can be copied elsewhere, or simply opened in Preview for subsequent copy/paste into a new Pages document.
If it is the default Pages '09 single format file ( a compressed and zipped Pages package format file) then the following Terminal command sequence will get you access to the Preview.pdf document inside the archive, and write it into the current directory location as Preview.pdf. You can subsequently access and process it as you did with Preview in the first paragraph.
unzip -j filename.pages QuickLook/Preview.pdf
If your document is a Pages v5 (any version variation), there is no PDF stored inside the document, and the preview.jpg is only the very first page. All of the document content is in *.iwa files that only Pages v5 knows how to open. I suspect that they are at minimum compressed, and probably seeded with an encryption key that only Pages knows about. At any rate, there is no way to recover your document from them.