If you can't see it, it may not be a big problem, but you can create a custom ProgressMonitor. This is a collection of callbacks, and so you can make a collection of callbacks that do nothing. I've used this idea successfully.
Note that the convert to/from PDF handler runs a whole series of separate tasks, some even may be in different processes (e.g. Word conversion). Some of these may show UI or progress which is unsuppressible; it depends on the conversion type. At least PNG happens entirely inside Acrobat.