Ascii85
Four bytes become five characters.
Where it came from
Ascii85 (a.k.a. Base85) was invented by Paul Riddle for the btoa utility and adopted by Adobe in PostScript and PDF. The idea: 85 printable characters is enough to represent 32 bits (4 bytes) in just 5 characters.
You meet it inside every PDF stream — encoded image and font data is wrapped in 〈~ ... ~〉 Ascii85 delimiters before being decompressed.
How it works
Take 4 bytes (32 bits) and express them in base 85, emitting 5 characters. That is ~25% overhead versus Base64's 33% — denser, but the alphabet includes trickier punctuation.
Variants differ: Adobe uses ! to ~, RFC 1924 (IPv6) uses a different alphabet, and "Z" often stands for four zero bytes. UWC handles the common encodings entirely offline.