Signature field has been setup to mark all fields as readonly after signing a document, and usually I save a copy of this filled document to another folder not to overwrite an original. However sometimes I click Save not Save As which overwrites it. Is there any way to protect an empty original form? A pdf can't be readonly because it can't be signed.
On the file system set the file and the folder of the file as read-only.
You should always be able to clear your signature, which would unlock the fields. Right-click it and you'll see that option. Then save the file under a new name.
You could use a write protected folder from where you open the PDF. This can be the default documents folder of Windows 10 for example.