There are several methods for sharing and collaborating on files (Excel, OneNote, PDF, PowerPoint, Word, etc.) within Teams.
Chat: You can share a file directly in your Chat by dragging and dropping the file into the message box in the chat. Then, you can access the file via Teams by clicking on the Add a Tab (+) at the top of the chat and
FIles: From the files tab on the left-side toolbar you can view all files that have been shared with (or by) you. You can right-click on a file and select an action: edit in Teams, open in Browser, Open in Desktop or download.
Teams: You can share a file directly in your Team (or Channel) by going to the
Teams tab on the left-side toolbar. Once you have selected the Team you will see the
Files option at the top. From there you can select
+ New to create new folders and files. Alternatively, you can upload existing files to this section through the
Upload button.