I have a XML file with a tag <doc>str.pdf</doc>, in which srt.pdf is a file.

When i transform this XML into PDF (with FOP), i would that the
<doc>str.pdf</doc> would be replaced by the PDF file str.pdf.

How can i do that?

Well, you can't. Apart from other problems, merging PDFs
is not as easy as merging text files. You might want to
take a quick look into the PDF spec before ranting about
lazy developers:
  http://partners.adobe.com/asn/developer/technotes/acrobatpdf.html

In certain cases you can probably do something, like
using iText to merge the PDF file directly generated
from your XML with your existing XML. You'd need to
write a custom Java program, or perhaps FOP extension
elements.