How to interrupt or cancel process when it takes too long?

To interrupt the processing, you can use the ProgressChanged event.

Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();

searchablePDFMaker.ProgressChanged += delegate(object sender, OngoingOperation operation, double progress, ref bool cancel)
{
   Debug.WriteLine("Operation {0}, progress {1:F}", operation, progress);

    if (stopwatch.ElapsedMilliseconds > LIMIT)
        cancel = true;
}

searchablePDFMaker.MakePDFSearchable(output);