You are only able to upload photos from an iPad to the dropbox in your course. Other saved files from outside the photos folder cannot be uploaded.
The recommended way is that you email the file to yourself and then upload to the dropbox from a laptop or desktop.
You may also consider using the Textilus app for text based documents (essays / worksheets). The paid version can also work with images inserted into a document. Once it is created and you have completed your work, click the "Send to Photos" option.