From Kenneth K. in the mailing list:
I have a client application that convert pdf documents into single page tiff images. Since the user export large quantities of documents I use a multithreaded approach. Each thread loads it own document and converts the pages one by one.
The pdf file is serialized to the client thread and then each thread executes the following code. If I only use 1 thread there is no problem. I checked to make sure objects are shared between threads.