That ‘denied by Referer ACL’ message means it’s looking at the HTTP REFERER header.
The referrer (misspelled in the www spec as “referer”) is the website you came from.
If you click the link, your browser sends this site’s URL in the referer header and you get a 403 (not sure why their firewall has that rule but it’s still set that way).
If you instead copy the link and open it in a new tab, then there’s no referring site. Apparently that makes their firewall happy, so the it doesn’t block you and you can see the doc.
That’s the trick if anyone else wants to look at the (Chinese) hardware spec sheet from the link.