If your workflow involves sending loads of email attachments back and forth, the aptly titled Annotate Attachments in Gmail extension for Chrome can help streamline your operation.
After installing the extension and restarting Chrome, the next time you mouse over an attachment in Gmail, you'll see a button has been added next to the Download and Save to Drive buttons. It opens the attachment via the Annotate Attachments in Gmail extension, which lets you annotate and add comments to the file. The extension supports images and PDF, DOC, XLS, and PPT formats.
At the top of the attachment is a small toolbar from which you can mark up the attachment via pencil, arrow, rectangle, and eraser tools. You can choose a color and adjust the size of the point of your drawing tool. You can also add comments to the document. Each comment gets its own reference point on the attachment and allows replies. The toolbar does not feature an undo button, but the Command-Z keyboard shortcut performed the maneuver on my Mac.
After making your annotations and comments, hit the Save button in the upper-right corner and the extension creates an email draft. Instead of sending the file you just annotated as an attachment, the extension simply adds a link to the file. And any comments made on the document are listed in the body of the email (they are also available via their reference points on the file itself).
The recipient does not need to have the extension installed in order to open the link to your file. And recipients can add their own annotations and comments and then hit the Notify button to alert you that the file has been updated.
Files are stored in the cloud on the developer's servers. Regarding security and privacy, the developer, Framebench, states:
We promise that all your data is yours and that we do not read or store any of your email content other than email headers. We will not make any changes to your inbox. Our servers are hosted on the secure Amazon infrastructure and all our requests happen on SSL 256 Encrypted connections.