The "Copy" function in the Resource Library allows you to have a single Document appear in multiple folders (you can even set different permissions for each copy).
Simply hover over the Document you want to copy and click the ellipsis icon that appears in the top right > Copy.
You'll then use the "Destination" dropdown to specify the Folder that you want to copy the Document to and specify the Users that should have access to the Document using the below Permissions & Visibility options.
How Do I View All the Linked Copies of a Document?
You can view all linked copies of a Document on the Document's Details page by clicking the ellipsis icon in the top right corner of the Document> Details.
What Happens When You Edit a Document that Exists in More than One Folder?
When you edit a Document that exists in more than one location, you can specify whether to apply the edit to all locations or separate the files completely: