This PHP Script has an Anti-Leech feature turned on.

Make sure you are accessing this file directly from www.steam-brite.com

It seems you are trying to get it from https://steam-brite.com/literature/schematics/index.php?dir=Mytee%2FUser+Manuals%2F&file=KS500-UserManual.pdf

Your IP address has been logged.
3.145.176.228 (ec2-3-145-176-228.us-east-2.compute.amazonaws.com)

Here is a link to the directory index the file is in:
http://www.steam-brite.com/literature/schematics/index.php?dir=Mytee/User%20Manuals/