Documents are uploaded as "Images." ("Images" is one of the buttons in the green strip in the top part of the profile.) The upload page notes that these formats are acceptable: JPG, GIF, PNG, or PDF, and they must be 10MB or less.
So if you have a scanned document, it will probably already be in an acceptable format. If it's a Word document, convert it to a PDF.
Then complete the information on the upload page, and indicate that you have uploaded a source (not a photo).