Reducing or enlarging font size is a function of any web browser or word processing software. How you reduce the size of the print on your screen depends which program you're using.
In most browsers (Internet Explorer, Mozilla Firefox, Chrome, Netscape, Opera, and Safari) you can reduce the size of the font on your screen by holding the "Control" key (or "Command" on Mac) along with the minus key (-). To enlarge, hold "Control" and the plus key (+). To return your display to the default size, hold "Control" and zero (0).
In Microsoft Word, first highlight the text you wish to change. Then go to the size drop-down in your toolbar and select which point numeral you would like to use in the document. Alternately, with the text highlighted, you may opt to reduce to the next smallest size via key command "Control," "Shift," "<."
Use the same steps you would with Microsoft Word with most text editing software. There should be a numeric value in a field in the toolbar, and highlighting the text before navigating to and choosing a smaller value in that field will reduce the font size.
Tick the minus symbol in the toolbar in Adobe Reader to reduce the font size of any document you're viewing in that program. There is also an option to type in the desired size of the document within the window by percentage in Adobe Reader's toolbar.