ABCPdf library - parallel processing

Question

Have any of you tried to convert *.docx file into *.pdf using ABCPdf library in parallel loop?

Here is my method that is called in Parallel.ForEach loop

public void ConvertDocxToPdf(string fileIn, string fileOut)
{
    var theDoc = new Doc();
    theDoc.Read(fileIn);
    theDoc.Save(fileOut);
    theDoc.Clear();
}

It appears to be slow. I have 8 cores but only 6 of them are used. Average CPU usage is 18%.