Downloading will save the file to your browser's preferred download location. Downloading files could lead to versioning issues, therefore downloaded files will also be automatically "checked-out".
The checked-out function alerts other users that the file has been downloaded and therefore may be an outdated version. When checked-out, files can still be referenced and viewed/downloaded; however they cannot be deleted, as discussed earlier.
As discussed in Adding & referencing files on a worksheet, files such as PDFs and image files can be viewed in the browser without having to be downloaded; however, editable files such as Word and Excel files must be downloaded to be viewed. Despite being viewable, all files can still be downloaded via the Download action button.
To download a file/files
Head to the Files screen and navigate to the file(s) you wish to download.
Select the file(s) you wish to download.
Click Download if it appears in the action bar, otherwise, click the More actions menu button, then select Download.
A Download confirmation dialog will appear alerting you that the file(s) will be marked as checked-out. Select Download when you're ready.