When setting up the upload manager, make sure you set up the Advanced File Manager by creating a connection to a database. To setup the Advanced File Manager, open up the HTML Editor wizard (by clicking on an existing one and hitting edit or inserting a new instance) and click on the customize next to Global File Upload Settings. In the HTML: Site Settings dialog, make sure to set a connection in the Advanced File Manager section.
Using the simple upload manager, uploading pdfs and docs will throw that error.