How to clean MacOS? (Ad keeps popping up) I installed some video player and might accidentally clicked to install something in my Mac. Then I deleted the player from the application folder. Now, after several hours, the system pops out a small window (alert window, with accept and reminder me later button on the right). Safari and AppStore also pop out with the PDFExpert app page shown. 
I am wondering is there any place to fix this issue. I didnot find anything special in my Notification center nor any special app installed in my Application folder. Is there anywhere else I need to check to fix it?

If no, I might need to us my timemachine to recover.
I would run Malwarebytes Anti-Malware for the Mac and clean things up. It is free. Get it direct from Malwarebytes: https://www.malwarebytes.com/mac/