Thanks to Charles Hankey's guidance, I developed a program that merges like PDF's into one PDF.
The process runs every few minutes, so it's possible that an earlier version of the merged PDF is in use by a user.
I'd like to be sure that no one is using the merged PDF before writing to it, skip it if in use and do the merge in a subsequent cycle.

Using C# what's the best was to see if the target PDF is in use?