You cannot edit a PDF on macOS unless you have paid for a third-party PDF Editor. Apple's Preview is a PDF reader. The Pages application cannot open PDFs. If you have access to MS Word 16.31 or later, it can open a PDF and convert it to a Word .docx document that can be opened in Pages.
If you launch Pages, ignore the File Chooser dialog and look under File menu : Open Recent. Is the Pages document that generated that PDF Mail attachment in the list of recent documents? If yes, you may (but not necessarily) have found the document, provided it is not deleted. Pages also remembers the last document save location, so pretend to save a document and see what folder it is defaulting to on that save panel.
Next, if you know the timeframe that you created the PDF from the Pages document export, or even a part of the case-insensitive Pages document name (perhaps same as PDF), then you might be able to use Spotlight to find it using your variation of the following (> means after):
kind:pages created:>01/01/2024 name:barkingcat
and if after all of the above, you still cannot find the Pages document, then you can retrieve it from your Time Machine backup (if originally stored on your Mac) via /Applications/Time Machine, or by opening the PDF copy/pasting that content into a blank, open Pages document.