Viewing zoom problem

I will be on line any program looking at a file example: numbers program, just lookin at a file on pages, looking at e-mail message like I Sid any program I will be at 100% zoom or ever another % zoom. All of a sun it will reduce to lowers zoom. I it the zoom % I am to take the zoom up it will go up to that % zoom but it will do it again it will reduce again. Tried everything I know but it keeps doing it. I sure there is a correct somewhere on my Mac computer but I can't find it. Help