So one the hand, it seems that adding a button to open a pdf manual in the system's default pdf viewer software is quite doable, and would not require that much development effort. However, this would also reduce the manual's accessibility, and the user experience would not be as polished.
On the other hand, having a locked down manual reader in the browser seems like it would be the most user-friendly option. However it's also apparent that creating it would require non-trivial research, design, and implementation effort. Realistically, mcs and brade would be doing most of this work, although I'd like to help out as I could. Do either of them of the time or willingness to make this a priority? If not, is Lunar willing to compromise his strategic vision for the manual in order to make sure this entire project doesn't get placed on the backburner and neglected?