Hello,
sorry for the late answer. In the online tools the edited PDF is saved as a download: in Edit PDF, click the save icon (Save PDF) in the toolbar when you are done and confirm with "Save PDF". The new PDF is then downloaded by your browser, usually into your Downloads folder: https://tools.pdf24.org/en/edit-pdf
For forms with fillable fields, Fill out PDF is easier, you can type directly into the fields: https://tools.pdf24.org/en/fill-out-pdf
If saving fails, the editor now shows a message with the reason, for example when a firewall, VPN or a browser extension such as an ad blocker blocks the connection to the PDF24 server.