1. Go to the Tasks module.
2. Check the box to the left of all tasks that for which you wish to attach the document.
3. Click the Add Document/Form Letter button.
4. Choose from the options in the Add section as needed. 
5. Select the folder to save the document to in the Save to Folder section. Please note that even though you're associating a document to multiple tasks, the document is only stored once in a single location.
6. Browse for the document you wish to upload using the Choose File button.
7. Click the Upload button.

When you attach a document to a task, this creates a link to the document that is stored in the Files module. Any changes made to the  document (such as the name or content) that is attached to multiple tasks, will replicate across all of them.