1. Click Account and "Support Cases" from the tabs on the top.
2. Locate the case you wish to add the document to.
3. Click through to the Support Case Details Page.
4. Scroll down and click on the sub-tab "DOCUMENTS".
5. Click Browse or Choose File button to locate file.
6. Locate and choose the file on your local computer.
7. Enter a description for the file you wish to upload.
8. Click UPLOAD.
9. Wait for confirmation of uploaded file.

Documents can also be stored independently within Leads, Accounts, Vendors, Opportunities, and Support Cases. Simply go to the corresponding module's detail page and scroll down to click on the sub-tab for Documents.