Inline grading in Brightspace is one of the features people have asked for. We had it in Blackboard but it doesn’t exist in Brightspace, natively. One approach is to download the file to be graded, put inline comments in the file, and then upload it back up. Comments can also be put in the generic area as well.
That works, but perhaps a better way is to put the comments in the document while it is still in Brightspace. That is very do-able if you do it in the 3rd party application, Turnitin. You don’t even have to take advantage of the anti plagiarism features, you can just use Turnitin to provide inline grading. The 15 minute video below explains how to do it.