Within the Permissions tab, click the Content Access tab if it’s not already displayed. Then choose the roles you'd like to work with from the roles list panel on the right.

In the far-left column are the contents of your fileroom (click the arrows to expand the folders). The central columns display the settings for each role’s content permissions.
To change anything here, click a segmented bar and click Edit Permissions.

You can set a role’s permissions to cover specific contents of the fileroom, or the whole thing. If you set a permission at a higher filing level (e.g. a folder), then those same permissions apply to everything below it (e.g. all files in that folder) by default. However, you can manually set different permissions for these lower-level contents if you wish.
If a fileroom or folder has its permissions bar highlighted in gray (or green in Edit mode), that means it contains some custom permissions that differ from the default.

Once you’re done, click Save.

Example:
Gareth is a reviewer. Currently, his role’s permissions for the Accounts folder are set to View only. Now he needs to download one particular spreadsheet from that folder. Project administrator Petra approves his request and changes his permission to Download for that document only. Gareth still can’t download any other files from the Accounts folder; he can only view them.