Letting User Retain Access To Updated Pdfs

I have created a product in my store that is a downloadable PDF.
I have added the PDF file to “FILES TO SELL”.
I set it available to, Activation mode: “After full payment”.
I am able to get it working but I would like to know how to set the expiration date for the download.
What I would really like to do, if this can be done:
I would like to set it up so that when a user purchases the PDF they retain access to a particular folder.
I would like to have this folder set so that I can post updated PDFs for the one that I am selling and let the users that purchased the original PDF have access to download the updates.
Is this possible to do?