i wrote to a professor an email asking for a lor and he agreed and asked where to send it. i then answered: "Since I will need it in a few years time, I was wondering if it would be possible to send it back to me in pdf by email, so that I can reach out to you with it closer to the application and, If still retained, renew it to date and send it on my future ERAS application when needed. 
Please let me know what you think, and thank you again for your time and help. I am extremely grateful." he then replied: "I would hold onto it and will draft it when the time comes- when you apply
no worries". What should I answer to receive now at least any document that states i took a course with him?