As a viewer on Google Drive, you can basically just look at the file. You can't make any changes to it, and you can't delete it either. But, you can download the file to your computer or device, depending on the type of file it is. For example, if it's a PDF or an image, you can easily download it and save it to your device.
However, if the file is a Google Doc or Google Sheet, things get a bit trickier. As a viewer, you can't download these files as Microsoft Word or Excel files, but you can print them or copy and paste the content into a new document.
Can Google Drive Viewer Download Files?