According to my test, this should be a problem with the Graph Explorer tool. You can switch to postman or other API tools to call the api.
You need to grant the Files.ReadWrite.All
application permission to your application in Azure AD, then use the client credential flow to get the token, and use the token to call https://graph.microsoft.com/v1.0/drives/{drive id}/items/{item id}/content?format=pdf
endpoint. Then you will get the download URL, you just need to run it.
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