Když otevřu Adobe reader DC tak pozadí je šedé s malými znaky,takže to znemožňuje čtení.

Hi Donecek,

I've edited your picture so that it does not show private information as you did not state that the file was showing dummy information. This is a public forum, and there need no more information then necessary out.

Now for your problem:

You are looking at what is called the transparency grid. The grey area shows what on your file is transparent and would, in case of composing different files on one sheet show the background. This is not needed for standard operations. You can easily switch this off in the preferences:

My preferences are for Acrobat DC not Acrobat DC Reader, but I suppose that the option is located at the same place on the preferences screen. There may be less options, however, on your screens.

See also here: Viewing PDFs and viewing preferences, Adobe Acrobat . In Czech language: Zobrazování PDF a zobrazování předvoleb, Adobe Acrobat . (I suppose your original post was in Czech, if not please excuse my wrong guessing and change the cz in the url to your language).