Working fine here…
Are you behind a VPN or corporate firewall?
Nope! It's weird...
The requested URL /en_US/acrobat/acrobat_dc_sdk/2015/HTMLHelp/ was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
You can download the zip files.
The SDK download links seem to work, but at least some of the exploded ones don't. "Overview" for example.