Provided you are using Apple Mail as your default Mail client and not using it to front-end another Mail client, an open Pages document can use the Share button on the toolbar to Send a Copy to Mail. The Pages document is attached as I just witnessed with Pages v14.1 and macOS Sonoma 14.5.
If you want to send a PDF, then you visit the File menu > Export To > PDF > Send a Copy, and the PDF is attached to the Apple Mail compose window.
However, if you are on macOS 10.13 (High Sierra) it is Pages v8.1 and I don't have that to test. You will need an Apple Mail client as the primary Mail agent — if any Mail server will except that ancient security handshake.