I was just putting comments on a 3 page PDF stored in Dropbox. I observed some quirks that could get old fast:
1. The color of the highlight for the comment was nearly the same color as the text in the document - we need to be able to change the highlight box color
2. If I were unfamiliar with the document, finding a comment could be tedious - If you click on a comment, the document should scroll down to reveal the area where the comment was attached to the document.