PD4ML objects access any resource (shared image and font cache, for example) by thread-safe way. But PD4ML instance is not re-enterant: you may not use a single PD4ML object simultaneously from multiple threads.

If you need to run multiple HTML-to-PDF conversions in parallel, you need to create a separate PD4ML instance per converting thread.

The safest practice would be to delete PD4ML instance after each HTML-to-PDF conversion and create a new object when needed.