That is NOT something you can normally do in a PDF file. You would need to zoom in using the iOS zoom in gestures.
You may be thinking of eBooks which use the EPUB format. In those documents (totally different format), you can select the font size.
You can also change the font size in a web browser since its using HTML.
In a normal PDF form document you do have the ability to change the font. I am adding text to a PDF form in adobe reader and the default font size is too large and cuts off part of letters. Some of the text boxs are too small for the amount of text I would like to enter and making the font smaller would be incredibly useful. I change the font all the time when editing the form on my PC. I move the document to the iPad, open with adobe reader and I lose that functionality.