Hi @VilmaMirando,
DocViewer is the document viewer/annotater that is built into the Instructure cloud hosted version of Canvas (I am unsure if it is included in open source Canvas or not). DoViewer manifests in various areas of Canvas such as SpeedGrader, and Canvas files area, and when files are embedded in other areas of Canvas. There is no separate sign-in, download, or access for DocViewer, is it really just an integrated part of Canvas.
I hope this helps a bit!
-Chris
