Meryn Stol added this answer. Meryn Stol No, this is impossible with Preview. Also, I'm not aware of any Mac PDF reader that lets you do this. The font size is a fixed part of the PDF document, and so the PDF file needs to be altered in order to change the font size. OS X Preview does come with limited PDF editing capabilities (including annotation and highlighting), but changing the font size itself isn't possible. To change the font size you'd need to use a PDF editor or convert the PDF to a different format (such as Word or ePub, depending on your needs) first. See question on Quora - Meryn Stol