PDFTeX (formerly known as TeX2PDF) arose from Han The Thanh's post-graduate research at Masaryk University, Brno, Czech Republic. The basic idea is very simple: to provide a version of TeX that can output PDF as an alternative format to DVI. PDFTeX implements a small number of new primitives, to switch to PDF output, and to control various PDF features. Han The Thanh worked on PDFTeX throughout his Ph.D. research into typesetting, and the latest release includes facilities he wrote to support novel typesetting techniques that he was studying.
Since he completed his studies, Han has had little time to work on PDFTeX, and day-to-day support is provided by a team of experts (mostly in Europe). The latest sources are available on CTAN, and implementations are available as part of Web2C, as well as in the teTeX, mikTeX, fpTeX, CMacTeX and TeX live distributions A version (by the author of CMacTeX) for use with OzTeX is also available on CTAN.
A mailing list discussing PDFTeX is available; subscribe via the TUG mailman interface.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=pdftex