There are hundreds of tools that are available for taking screenshot for free but as said rightly they do comes with ads inside. However instead of installing any product you can just add an extension in google chrome and take fullpaged screenshot of any website.
Install LambdaTest Google chrome extension to take a full paged screenshot of any website from a set of browsers, their versions. You can also mark the screenshot and also get a sharable link. Read more about the tool here.