Whenever you open an asset (what we call a file, video, pdf, etc.), there are a series of icons in the top right hand corner that allow you to interact with the asset. Below is a list of what each of those icons do.
Icon Row on an Asset Screen
The icons in the top right allow you to interact with the asset.
The Annotate Icon (Pen)
The Annotate Icon opens the annotation function. There is an overview of how to annotate here.