How to close pdf reader

I am running Windows 8.1 (not a fan) and when I plot a pdf it opens in the Libre Office pdf reader which is fine as it it is fast and verifies that the pdf is correct. I cannot work out how to close this reader! What key stokes or actions close it? I’m obviousy missing something here so answers in words of one syllable word be helpful. Thanks.

I don’t LO has a build in PDF-Reader. I tried it on Win7 and after export it opened the default system PDF reader (here Adobe).

If you don’t see the red X in top right corner, try Alt+F4 to close it. Or right click in the top bar and select Close. Sometimes it might be in Full Screen w/o top bar, then press F11 to get back to a normal view.