The PDF viewer uses Google Docs to inline your forum's hosted PDFs on demand.
This is the URL that is accessed when clicking the "Show PDF"
1. your browser must not be blocking access to the 3rd party website, google.com.
iirc, DuckDuckGo browser blocks this unless you've updated a setting to allow access to google services.
iirc, Configuring any other browser's Privacy & Security settings from "Standard" (default) to "Strict" will also disable access to 3rd party websites in the same way that it disables access to ads and embedded content where its domain does not match the source's domain.
2. your forum must be using Full URLs, so that the URL can be opened by the PDF viewer.
Control Panel > Paths & Database
3. it may be a docs.google "hiccup." try refreshing the page. yes. seriously.
could easily happen when trying to display the PDF before it has had a chance to upload to google's servers. if anything is delayed within the process, you might be notified with an option to view it directly on google's website or that it could not be displayed at all. a large PDF may cause this.