How could I close two pdf files I was reading ?

If an app on your Mac stops responding and you can't quit the app normally, use Force Quit to close the app.

Press these three keys together: Option, Command, and Esc (Escape) or choose Force Quit from the Apple () menu in the upper-left corner of your screen. 

More details here:

https://support.apple.com/ro-ro/HT201276

If Preview had not hung up on you, press option, and then select File menu : Close All. If you do not then formally quit Preview, it will disappear from the Dock, but continue running in the background when you switch to another application.

When that happens you can go to the Apple menu at the top left of. your screen and select "Force quit..." Then force the offending application to quit.