Managed to find the solution!!!
encoded_file_str = File.read!("sample-1.png") |> Base.encode64()
AWS.Client.create("XXX", "XXX", "ap-southeast-1")
|> AWS.Textract.analyze_document(%{
"Document" => %{"Bytes" => encoded_file_str},
"FeatureTypes" => ["TABLES"]
})
Thanks @benwilson512 for the hints!!!