Tried this on a whim and it worked. Normally Typewriter in Acrobat 8 (Pro only?) is intended to fill in forms in pdf files. However, you can use it ANYWHERE in any document to make notes. Handy for simple notes, if you don't want or need the "Sticky notes" or text box, or other features that enable more text style choices.

For example, I might save a scanned receipt as a pdf for business records, but want an annotation of some kind that can be read without complications by another party. Or perhaps just want to note payment info, etc.

Select the Typewriter tool, click once ANYWHERE on the page and type. When you're done, click again anywhere to conclude. The note is in a box (with no border that prints) and can be moved wherever you want it on the page.

To edit the note, select the Typewriter tool, double click to select all, or place a cursor.

To delete, select Typewriter tool, click on the note and - you guessed it - "delete."