JBIG2 is not specific to fax machines... Adobe Acrobat proposes CCITT group 4 (which is probably the most widely supported compression type on fax machines, by

JBIG2 is not specific to fax machines... Adobe Acrobat proposes CCITT group 4 (which is probably the most widely supported compression type on fax machines, by the way), and two flavours of JBIG2, when compressing scanned monochrome documents. Whence the need to support this in PDF files (JBIG2 is accepted since PDF version 1.4). JBIG2 is _not_ a legacy format, even if the use of its lossy variant must be heavily discouraged because of possible issues (e.g. invisible corruption of scanned documents).