PDF Authoring tool, suggestions?

[Modulok]

I'm looking for recommendations for a BSD-friendly, PDF authoring tool
(Not a viewer, see below.)

I have an image of a signature that I need to put into a PDF document.
I converted the original, un-signed PDF document to a raster PNG, then
used GIMP to place a photograph of the signature where I need it. I
then converted the PNG back to a PDF using ImageMagick's 'convert'
utility. The problem, is the PDF is now HUGE. Both in dimensions as
well as file size. It went from 28kb to 292Kb. There must be a better
way to do this.

Suggestions?
-Modulok-