Free source for certain user manuals: FCCID.io
 

https://fccid.io/

Example
https://fccid.io/BGBHS-U776 has user manual, spec sheet ("tentative" features), external photos, internal photos, and PCB schematic diagrams (plus stuff not interesting to us, like internal communications about the EMI testing).

The tentative specs go into more detail than typical advertised specs, but I don't think most models on the site include these.

Problems
Last week, they had some hosting issues. I couldn't get anything besides the main page to load. Because of this unreliability, I suggest saving anything you want from there as soon as you find it.

Searching
If looking for a specific model number, try a Google search like "HS-U776 site:fccid.io". This doesn't work well for Panasonic, because their FCC IDs are arbitrary. In some cases Google will still find the page, because the model number is written in text somewhere. In other cases, like PV-9450 (FCC ID: ACJ927122AH), the model number is only included as part of the scanned documents, so it's unsearchable.

Interesting site. :)

Do us a favor: add anything you find there to this site, so that it's long-term archived for others.