SDoP is a Simple DocBook Processor. It is a typesetting program that reads DocBook XML and outputs the typeset pages as PostScript. This can easily be converted into PDF form by the ps2pdf command that comes as part of the GhostScript package. SDoP supports most of the subset of DocBook that is known as "Simplified DocBook". Illustrations can be encapsulated PostScript, Scalable Vector Graphics (basic support only) or, if the relevant libraries are installed, JPEG or PNG images. You can read the SDoP manual on this GitHub site.