There is no written specification, it’s just serialized C++ objects. I think each line is one Residue object but I’m not sure. The only program that can read/write the format is Rosetta itself – generally one “extracts” interesting models from the binary silent file to PDB for further use. There is an executable for the purpose (extract_pdbs or similar).
Efforts have been made to make the system robust against code changes, but broadly only the version of rosetta that wrote a binary silent file can reliably extract from it. (It also needs to be on similar machine architecture).