Below the module pagelet click the "Add New Document" link and fill out the textboxes. If you intend to post a link to a file on your site make sure to upload it to an appropriate folder in the "Links" segment.

Clicking the "Update" link at the bottom of the page completes the task.  Optionally, you can indicate document Category before saving the changes.

If you are an administrator of the site you may find it better to split the task into two steps.

Step 1: Uploading the file with the Admin/File Manager page followed by
Step 2: Linking to the already uploaded in Step 1 document by simply selecting an appropriate directory and file in the Links segment of the Add New Document interface.