PDFBeads is a small utility written in Ruby which takes scanned page images and
converts them into a single PDF file. Unlike other PDF creation tools, PDFBeads
attempts to implement the approach typically used for DjVu books.
Its key feature is separating scanned text (typically black, but indexed images
with a small number of colors are also accepted) from halftone pictures. Each
type of graphical data is encoded into its own layer with a specific compression
method and resolution.