Thanks for your inquiry. It seems you are using quite old Base URI. Please use https://api.aspose.cloud/v4.0/words instead of https://api.aspose.com/v1.1/words. I have tested the scenario using GetDocumentWithFormat API and unable to notice any issue.
Furthermore, please check Aspose.Words Cloud blog post of API Version 4.0 for more details
// First get Access Token
// Get App Key and App SID from https://dashboard.aspose.cloud/
curl -X POST "https://api.aspose.cloud/connect/token" -d "grant_type=client_credentials&client_id=[AppSID]&client_secret=[AppKey]" -H "Content-Type: application/x-www-form-urlencoded" -H "Accept: application/json"
// cURL example to convert RTF to PDF
curl -X GET "https://api.aspose.cloud/v4.0/words/sample.rtf?format=pdf&outPath=sample.pdf"
-H "accept: application/xml"
-H "authorization: Bearer [Access_Token]"