Absolutely! Whenever you or a visitor to your document are viewing your document, there are "Add Comment" buttons at the top and bottom of the document:

If you are editing your document, you must first switch to viewing it by choosing the "View or Print" button.

If you are looking at a list of documents and folders (your own, or someone you are visiting), just click on the the name of the document and it will be placed in the viewer for you.

Then click on one of the Add Comment buttons, type your comment and choose Save and Exit to save your comment. Your comment will be appended in most recent order to the bottom of the viewed document, along with your KeepandShare user name.

If you are the owner of a document, you can delete any comments you don't like by choosing the blue delete command that appears next to each comment.