Azure Data Factory "base64(string)" expression

Hi,
I have a scenario where I am reading .pdf file from azure blob storage, converting it to base64 and then pass it on to an api which accepts a base64 encoded message.
I am doing some internal testing to see if my base64 message is correctly formed and I am running into an issue. The base64 message that is created by data factory pipeline, when decoded and saved is resulting in an empty pdf document.
The expression in "Base64 PDF Letter" shape is ==> @base64(activity('Get PDF').output.Response)
I also verified by uploading my base64 message to following url. The resulting pdf is blank.
https://base64.guru/converter/decode/pdf
The pdf i am reading is a valid file. I wrote a console app to base64 encode and decode using c# just to check the difference in base64 message created. It was very different.
I am not sure If i am doing something wrong here. Can you please help. Thanks.
Regards,
Adil