We're an architecture firm and we use Dropbox to distribute our drawings in PDF format.
In the past clients, engineers and building contractors would print off our drawings, mark them up using a red pen with comments and scribbles and then mail them to us so we can incorprate their comments.
These days we distribute drawings by PDF and if they have the technical means, clients, engineers and building contractors will mark up our PDFs directly and then email them to us.
It would be fantastic if they could mark them up directly in Dropbox (via web browser and mobile apps) without needing to download the PDF, open it in some other software and then email them to us. And it would be even greater we could both mark up a PDF live while talking over the phone.
Currently Dropbox has a comment feature but this is not enough for marking up drawings. It's more suited to text documents. For marking up drawings we really need to be able to draw lines and other shapes + add text directly to the drawing.