If you have a PDF document and you want to increase the font size in that document, you'll need to go to the application that created that document, increase the font size and then re-export it as a PDF.
Please keep in mind that a PDF is a digital printed document. Think of taking a printed page and wishing to increase the font size, how would you do that?
At least in a PDF you can zoom in. With a printed page you can hold it closer to your eyes and/or use a magnifying glass.
Sorry, I really am not trying to be snide but there is no way to do what you wish.