Links in the User Manual

When I hover over a link in the PDF user manual, it shows a link similar to this.
helpcrafter://Page42
When I click on it, I get a popup stating:
There is no application set to open the URL helpcrafter://Page42.
Search the App Store for an application that can open this document, or choose an existing application on your computer.
In other PDFs, clicking a link will take me elsewhere within that PDF. Is something broken here?
Using a PDF app called Skim.
helpcrafter://Page42
When I click on it, I get a popup stating:
There is no application set to open the URL helpcrafter://Page42.
Search the App Store for an application that can open this document, or choose an existing application on your computer.
In other PDFs, clicking a link will take me elsewhere within that PDF. Is something broken here?
Using a PDF app called Skim.