Resources for *roff internals and PDF generation

[Olle Lögdahl]

Hello,

I understand this mail is a little outside the scope of this 
mailinglist; i just had nowhere else to ask. Does somebody know any good 
resources for understanding the internals of groff/roff? Mostly curious 
about the PDF-generation part. Any resources may be helpful (online, 
books). I already have the PDF-1.7 specification, but something more 
specific on document generation and easier to digest.

Thanks in advance,

Olle Lögdahl