When a file replace feature is used, the request URI going to Varnish for cache clear contains domain along with the full file path like 'https://www.abc.com/sites/default/files/abc.pdf' which is not valid for cache clear, the correct URI should be '/sites/default/files/abc.pdf'
Install and configure the Adv Varnish and replace an existing file
On File replacement, RequestURI should go without domain for Varnish cache clear.