1. Open a board. 
2. Find the tile you would like to link a document to and open its menu by clicking on the menu icon (3 dots).
3. Click the "Pencil" icon to bring you into Edit Mode
4. Find the box that contains " http:// "
5. You can type in or paste the link to your document. 
6. Once you are done click the blue check mark or hit the "Enter" key on your keyboard to save.

Note: If your Virbeya board is connected to a SharePoint site you have more options to upload/attach documents to tiles. See SharePoint Integration for more info.