The PowerPoint filter is in charge of importing and exporting from/to PowerPoint binary documents (.PPT) into Impress. If you are a looking for the PPTX files from PowerPoint 2007, please go to the Office Open XML.
The PowerPoint filter is maintained by Sven Jacobi.
The import code is scattered between the svx and sd modules. Relevant files are in:
The export code is contained in sd/source/filter/eppt
PPTWriter is the main class that will write the binary stream. Its implementation is spread accross eppt.cxx and epptso.cxx.