Steps uninstall the A5 Documents package: Setup > Installed Packages > find A5 Documents app > click "uninstall" link next to A5 Documents package. When you click to uninstall, a notification may pop up that you have a few more steps before you can uninstall package (for example if you already added a custom button or link to your object pay layout then you may have to remove the button from page layout before uninstalling.
If you are not able to see "uninstall" link or unable to click the link then your Salesforce user may not have sufficient permission access to uninstall an app package. You can have a Salesforce admin do it for you or the original user who initially installed the package from AppExchange.
If you also need to cancel your app subscription, you can refer to this article.