reading existing pdf files and password protect them

[Deepak kumar]

Hi All,

In our payment system, we are getting some invoice pdfs generated from SAP system and stored in aws. 

So basically i have to download these pdfs from aws and make it password protected and send to the customer via email attachment. Using "prawn" we can easily password protect newly generated pdf files but after a lot of search also i can not find a way how we can add password to existing pdf. 

Is it possible to achieve ? 

If not is there a way i can completely copy existing pdf content and create a new pdf based on the existing copied content ?