Is it possible to use the PDF Doc from Template smart service to write a value i

I tried this out and observed the same behaviour. However, out of curiosity, could you share the use case for this?

My use case is that we are having the user digitally sign the PDF, and they need to save it and re-upload the signed version. We want to put an ID on the document that we can then verify with the Parse PDF Plugin that is invisible.

I worked around this by instead creating a read-only visible field, that had the text color set to white, and the font set as small as it could go, so it would be effectively invisible.