If the form doesn't provide a means for changing the font size, there's not much you can do. It may be that the font size is set to Auto, which means that Reader will auto-decrease the size . Dec 1,  · Fortunately, changing font size in Adobe PDF fillable forms is a relatively simple process. Here are the steps to follow: Method 1: Changing Font Size in Adobe Acrobat DC. Dedicated Experts · Wide Variety of Brands · Expert Consultations · Industry Leading Partners.