managing several page formats in windows with Java/PDF

[Pif]

Hello, in my intranet, I need to generate a PDF file that could use
two different paper trays.

In this file, I append several documents. Each one contains a letter
that should be printed in a white paper. Then the bill is joined that
should be printed in a "pre-printed" paper.

So these two papers must be places in two different input trays of the
printer. Do you have an idea of how I could make possible that my
document is well assembled in the ouput tray ?

The PDF document is generated using Java Servlet, XSL/FO and FOP.

Thanks a lot for your help.