I had some faculty interested in allowing students to scan and submit handwritten work to their online courses. Creating a Canvas assignment that only accepts JPG pictures that students take with their phones wasn't good enough because instructors can't use the SpeedGrader (DocViewer) to annotate student work. [Update: as of the June 2, 2018 Canvas release, you can annotate images in DocViewer now. There still may be some advantages to converting scanned images to a single PDF, however.]
So the solution we have now is for students to use one app on their phone to convert photos of their work to a PDF document and then submit that PDF to Canvas via the Canvas Student app. One popular app for creating PDF scans is Tiny Scanner [UPDATE: Don't use CamScanner]. I know many have already known about this solution for years now, but I couldn't see it documented anywhere on this site.
Luckily, someone already made video instructions for this workflow on both Android and iOS devices. I would also though recommend students turn their phone sideways, so that the images in the PDFs aren't rotated incorrectly in the SpeedGrader (which doesn't have a button to rotate documents or images back to the correct orientation - vote for the feature here).
FlipGrid - Video Discussion Option
Another alternative technique that is growing in popularity is to let students use FlipGrid to submit videos in which they describe and comment on their work. See