You didn't mention why you wanted an offline version, so depending on that this may be too meaty of a solution for your needs. Unfortunately there isn't a quick solution for what you are asking for.
You could run a SharePoint virtual machine on your computer and then back up and restore your site to the VM. This will give you offline capability and a replica of the site, at least from the time that you did the backup.
I am not sure if you are 2010, 2013, 2016, Online but what version you are running will dictate how much work you will have to do to get a VM up and running. And I don't believe you can create a VM for SharePoint Online. With that you would have to save down the pages manually. For 2013 and 2016 there are VMs already created that you can download and use. You can probably find an old 2010 version around somewhere. The Microsoft hosted one is no longer available.
You also need to have admin rights on your existing site in order to do a backup of the SharePoint site.
There are a ton of VM resources, especially for more recent versions, that you can reference. Here are a few to get you started.