PER provides a much more compact encoding than BER. It tries to represent the data units using the minimum number of bits. The compactness requires that the decoder knows the complete abstract syntax of the data structure to be decoded, however.
There are two variations of packed encoding rules: unaligned and aligned. With unaligned encoding, the bits are packed with no regard for octet (byte) boundaries. With aligned encoding, certain types of data structures are aligned on octet boundaries, meaning there may be some number of wasted padding bits. Unaligned encoding uses the least number of bits, but presumably at some cost in processing time.
Documents: ITU-T X.691, ISO 8825-2.