If files have been shared with you in a channel and you need to download them, hover on the title of the entry (which will highlight the entire entry) and then click on download arrow that will then appear:

Check that you have pop ups enabled on the browser and then you should see a zip file begin to download. Once it’s finished downloading you will be able to extract the contents and view the files.

You can also download individual files by clicking on the arrow to the left side of the title of the entry, then selecting the drop down arrow to the right of the title of the individual document and selecting Download: