Addon Manager can’t be downloaded or directly installed

In the admin page for ‘Newsletter’ in my WordPress installation
/wp-admin/admin.php?page=newsletter_main_extensions
if I click on ‘Download addons manager’ I get a web page of gobbledegook.
Also, if I go to the support page for the Newsletter Addons Manager
https://www.thenewsletterplugin.com/documentation/installation/how-to-install-the-addons-manager/

and click on “Click on this link to download the official Addons Manager for Newsletter” I get a web page of gobbledegook.
Could you help please? I believe that I should be receiving a download from your website, but it is rendering the file to my web browser as if it were a web page. I tried two different web browsers.
Cheers,
Mark

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