I recently saw an archived idea (listed at the end of this listing). I am sad to see that it was archived with only 42 votes (I just leaned about voting today...brand new here). I would like to reemphasize a use case.
In today's paperless classroom it is useful to gather information from students through writing. Specifically in the K-12 arena, where we still sometimes ask students to fill out worksheets to show certain level of mastery of fact based work such as mathematics and grammar. While K-12 education should be striving to change from multiple choice tests and simple drill and kill to cognitive mastery, there are still cases where this type of assignment can be useful at reemphasizing skills. Furthermore, while speed grader does give the ability to the teacher to quickly click through rubrics or embed notes there are times teachers would like to "write" directly on student submissions. Adding in an annotation feature helps in both of these situations. Annotation features could also be used for signatures of documents passed back and forth for things such as field trips and other permission based files teachers may wish to send home with students.
For younger children especially, it is hard to advocate for using separate apps. Having one application that does viewing of pdf files and annotation would cut down tremendously on downtime used to submit and view documents. It would also tie well with the offline files feature that Canvas has already implemented for uses where internet is not reliable or homes which have no internet.
I would like to see the archived idea reopened for a vote. There are many competing products out that already do this sort of annotation. Our school is actually moving from a different product to Canvas because we want a more full blown LMS for online courses in the future. My teachers are all still under beginning phases of training and would all vote for the feature if it were opened up again. That would be 80+ more votes for implementation.