IdCoderQuotedPrintable.pas contains Classes and Methods that are used to implement encoders and decoders using the Quoted-Printable encoding scheme as described in the the Internet Standards documents: 

RFC 2045, Multipurpose Internet Mail Extensions (MIME), Part One: Format of Internet Message Bodies, Section 6.7 Quoted-Printable Content Transfer Encoding  

RFC 2047, MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text  

The Quoted-Printable encoding scheme is intended to represent data that largely consists of octets that correspond to printable characters in the US-ASCII character set. It encodes the data in a way that minimizes the chances of the message content being modified by a mail transport. The quoted-printable encodings transforms input into characters in the "7bit" range.