Multithreaded office to PDF Converter using office with interop

I need a small, reliable utility that takes Microsoft office files (.doc and .docx, xlsx ..) and turns them into PDFs by calling the Microsoft Office Interop API from either C# or unmanaged C++. The conversion has to run in the background using a multithreaded approach so several files can be processed at once without blocking the main application.

Here is what matters most to me:
• The tool must spawn or reuse Word instances safely, release COM objects properly, and never leave orphan processes.
• It should accept a list (or queue) of input paths, spin up worker threads, convert each document to PDF, and write the finished files to a designated output folder while preserving the original file names.
• Error handling needs to be graceful—if a document is locked, missing fonts, or otherwise fails, the thread should log the issue and continue with the next job.
• A brief README that explains prerequisites (Office version, interop assemblies, any registry tweaks), how to build, and how to invoke the executable or library from another process.

Deliverables
1. Complete source code in C# or C++ with clear project structure.
2. A compiled executable (or DLL) demonstrating the conversion running on multiple threads.
3. README / build instructions and a short test script or batch file that shows sample usage.

If you have previous experience wrangling Office Interop in multi-threaded scenarios, this should be straightforward. Let me know which language you prefer and any quick ideas you have for keeping Word instances under control.