Tom,
Inside of Canvas you should be able to message all of your students through the inbox link on the left of your screen. On the mobile apps I believe this section is at the bottom. Once in your inbox you should be able to create a new message from the "pencil and paper" icon in the top right of the screen. When the new dialog opens, select the course you want to address and then in the "to" field you should be able to select "students" and then "all students." You can attach files using the paperclip at the bottom of this composition window. That should send the information to any student in any section of your class.
Typically what I would do in this situation (If I did not need feedback from the students) would be to create a class announcement. Here you can put in a topic (like an email subject line) and body text (like an email message) and then use the attach link to upload your files. The default will send to everyone in the section but you can also break this up to send to only certain sections or only certain students. And you get the bonus of allowing comments so you can turn it into a group discussion.
Hope this helps. My teachers and I all do things a little differently based upon our workflow and needs. There are lots of different ways to communicate with others through Canvas!
