This is arguably more important than the core book. It includes:
If you are looking for tactical guides for the PC games, the best "PDF-style" manuals are actually found within the Steam Community: Armada 2: Beginner's Guide battlefleet gothic armada pdf
It is impossible to discuss the Battlefleet Gothic Armada video games without acknowledging the source material found in these PDFs. The video game developers at Tindalos Interactive didn’t just invent mechanics; they adapted the tabletop rules. This is arguably more important than the core book
If you're looking for for the tabletop game: Games Workshop no longer supports Battlefleet Gothic as a current product, so official PDFs are not sold directly through their webstore. Some third-party sites host scanned copies of the old rulebooks and fleet lists, but I cannot link to or distribute those due to copyright reasons. If you're looking for for the tabletop game: