Why downloaded files are not signed in Dropbox Sign

We suggest waiting for callback events within your app. These callback events are POSTed to your callback handler throughout different stages and/or errors within the signature request process. For more information on callbacks, please see our Events and Callbacks Walkthrough.

You'll want to wait for the signature_request_all_signed event to POST before downloading the signed document using the Get Files endpoint. This callback event fires off once everyone has signed and the document is ready for download.