You cannot pause an exam.
On the real Project Management Professional (PMP)® Exam, you will have a predefined amount of time to answer all the questions. If you take a break, then the clock "keeps ticking". We have built The PM Exam Simulator™ to reproduce the "real" experience as closely as possible, so therefore, there is no option to pause an exam.
If you only have a limited time to take an exam (for instance, you have 1 hour over lunch that you want to spend taking a "short" exam) then use the Quiz mode.