Hi Wolfgang, i've made the stress test and it works fine in multi-threading.
My hardware is:
Windows2008R2 x64 (Virtual Machine with VirtualBox)
XML fetched from MSSQLServer
XSL fetched from MSSQLServer and keep in memory
Class called in WS (asmx)
Stress with SOAP-UIx64
Total 724 PDF's, two images in each page, 5 pages per PDF. Near of 2.4 files per second.