What kind of text are you referring to? If it's static text that's a part of the document then you can't decrease its size in Reader. The best you can do is to zoom in or out to change how it is displayed on the script.
The method you described only works for comments and some form fields.
Thank you for your prompt reply. It is text that I am entering into a form. The font is too large and I want to decrease it to fit into the box. Does Adobe Acrobat Reader DC allow for such a function? If not, what Adobe Acrobat product does?
You can only do it in Reader if the author of the form set that field as having Rich Text Contents.
Any version of Adobe Acrobat that isn't Reader (ie, Acrobat Pro or Acrobat Standard) can change it.