Create a new Indesign or Illustrator doc that's the exact dimensions of the pdf.
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Use Place in Indesign or Illustrator and place the pdf in the doc. If you wish you can lock the pdf or create a new layer so the pdf doesn't change position. Then insert the pics, text, whatever. Then save the Indesign/Illustrator doc in pdf format.
Using OSX 10.3.9 on a G4