A PDF document basically contains exact coordinates for the location on the page for each individual character. If you increased the font size without changing those coodinates, the characters would overlap one another, and that would unreadable.
I think it would be better to know why you want to do this: is the PDF displayed too small to be read on your device? You have "device: none" by your profile, so it's hard to guess what your needs are. If you are just reading on a computer screen, surely you can use your PDF software's zoom feature to make it look bigger. Or are you trying to make it better when printed?
If you're reading on a device, probably it is already zooming it to the largest it can be zoomed and still have a page fit on the screen. Your best bet to make the individual characters look larger without affecting the structure of the PDF would be to remove the whitespace margins from the borders of the document, and or cutting the PDF pages into chunks appropriately sized for your device.
A program like
sopdf is ideal for this if your device screen is similar to a Sony Reader's (which many others are). You might want to give it a try.
What other options there are would depend on your device, and whether or not this PDF is the sort that can be converted well to another format, which would depend on whether it's mainly plain text, or has things like mathematical symbols, images, columns, tables, etc., in it. (These make conversion from PDF to another format difficult.)