good library for pdf

[Rob Cowie]

Take a look at www.reportlab.org. The ReportLab library includes a
graphics module that might well do what you need. I'm not sure at
present if it allows one to set alpha-channels to achieve transparency.

Also, if you have access to a mac running OS X 10.4, the Automator
application has a prebuilt action that applies a watermark to pdfs.
Transparency, size and placement are all editable. Obviously, no use if
you want to do this with Python, but it might suit your needs.

Rob C