Answered Restrict Access to Folder via Modx Manager

This was actually discussed here on the forum about 4 days ago. Unfortunately the search here does pull a lot of very old posts by default at the top of the list, so you'll want to make sure to select a more recent start date in the advanced search to weed out those old posts.

https://forums.modx.com/thread/81581/limit-access-to-downloadable-file

You will want to digest completely all the information about MODX Security and permissions as well.
These should give you a good start:

http://rtfm.modx.com/display/revolution20/Security
http://bobsguides.com (click on MODX, go to security section)
http://modxpo.eu/schedule/sessions/modx-revolution-security-permissions-system - video of Bob's session in Utrecht regarding MODX Security
http://bmv-interactive.com/home/modx-acl-tutorial.html

I would also recommend getting Bob Ray's MODX: The Official Guide, and don't forget to make your allDocs resource group which will contain every resource, make sure to add yourself to it. I recommend installing the DefaultResourceGroup plugin and setting it up for the allDocs resource group so you won't need to manually add resources to that group when they are created. It's the first thing I install and setup now that I know better :~}