I'm a newbie to Sharepoint and I was wondering if it's possible to upload an Excel document and stop other users from downloading this file. I would like to have this file readable only inside Sharepoint and nothing else.
If a user has Read permissions to a file, then they will be able to download it. You can specify 'View Only' permissions, which will prevent a report (rdl) from being downloaded, but won't stop a user from downloading a word document.
You can look into Information Rights Management, which can specify for an individual document who has the rights to have it on their computer. It isn't an easy thing to implement.
If you want to restrict users from downloading the file (that is what i understand from the title of this post) you cannot do it through only changing the file permissions