What you describe is HTML document title produced by WordPress itself. The theme has no control over that.
I suggest trying a SEO plugin (such as Yoast SEO) as usually those offer control over HTML document titles. Though I’m not sure if you will be able to control the WordPress archive page titles.
However, I’ve checked the archive page titles on theme demo website and I don’t see the “archive” suffix there (please note that I do not use any SEO plugin there, just plain WordPress titles). Maybe you are already using a plugin that actually adds the “archive” text for you? Here you can check the theme demo pages:
I’ve actually went ahead and checked the Yoast SEO plugin settings and it seems you are using that plugin on your website. As the plugin actually adds the “Archive” text there. Here is a screenshot of archive page title tag settings for custom post types: