If you have a PDF form field with too small or too big a font, changing the PDF font is an important step to filling in the form so that it is readable. The process depends on whether you have a free or paid version of Adobe Acrobat, but in most cases, you can make the changes to fill in the form so that the content fits into the field. In some cases, you can't make any changes; if you run into this problem, you may be able to make changes if the author of the PDF adjusts the options.
Fill & Sign controls most of the form fields for PDF documents, explains Adobe. This feature is particularly useful if the author of the document didn't add form fields to the document but left spaces for you to fill in the content. Either click the "Fill & Sign" option on the toolbar to the right of the document, go to "Sign" and then "Fill & Sign" from the menus along the top, or go to "Tools" and then "Fill & Sign." Choose "You" when asked who will be filling in the form, and then you'll have everything you need to adjust the font size. This is the only way to change the font size in Adobe Reader.