Filed locked after calling FileRef.CreateFromLocalFile

Using Export To Docx I want to delete the original PDF file after calling the following:

FileRef sourceFileRef = FileRef.CreateFromLocalFile();

After Creating PDF from local file. I want to delete the original file but a lock remains on the file.

When I try to delete the file with File.Delete, an exception is thrown that there is a lock on the file from another process.

I cannot dispose of the the FileRef and even after setting the object to null, the lock remains.