Adobe have recently started shipping Adobe Reader X (pronounced ten) with some interesting new features. In previous versions of the Reader it was only possible to add comments or editing if the author of the document had switched commenting on through Adobe Acrobat Professional.
This feature was designed to enable reviewers to comment on draft documents and send them back to the author. However, I have found it very useful as a way of adding notes for myself on the document, in essence an electronic version of scribbling on a hardcopy. Nearly all of the documents that I receive do not have this facility switched on so I had to use Acrobat to modify the file to allow me to do this.
In Reader X there is the facility to add highlighting and sticky notes to any document. This means that I can electronically scribble on any PDF document I receive and store the results.
I see this not only as a usability feature for everybody but in particular as an accessibility feature for people who are unable to scribble on hardcopy. This may be of great assistance to people with musculoskeletal disabilities.
By the way, because this is a new version of the Reader it will not get automatically downloaded to your machine; you have to go to www.adobe.com and request a download of the new version.