This is a custom format, ideal for packing many different bitmap font sizes/styles/etc into a single binary buffer.
The buffer has the header P B M F (Packed BMFont), followed by a uint8 describing the spec version number (currently 1). Then Int16LE to describe the number of BMFont files contained in this binary.
Each file represents a distinct size and font. Each file chunk starts with a Int32LE, the size of the chunk, and then the chunk itself (as per BMFont binary file spec).