To add a document to assignment instructions, go to the assignment editing page. 

Click the Insert Stuff option.

Click the My Computer option, then click upload and locate the document you wish to attach. Once you have selected the file, click upload.

Under Link Text, add the text you want associated with the item - what you type in here will be how the file will show up in the assignment.

Under Alternative Text, add a description of the item. This will help make the file more accessible to students. 

Once you have the settings you want set. Click Insert.