First you have to realize that most PDFs like restaurant menus on the internet are not text, rather they are an image like a JPG, or GIF. As an image it is easily scaleable, but you scale the entire image and lose all the automatic page wrapping. It is like zooming in. If you have a trackpad, almost any application in OS X will do that quite nicely using gestures. (Spread the thumb and forefinger apart on the trackpad to zoom in or squeeze the together to zoom out.) That is exactly what I do when the text of a page is too small no matter what format it is in.
If you want the formatting, line wrapping
etc to adapt to the larger font size that is a very different thing altogether. There is no automatic formatting like line wrapping in PDF. However, there are applications that can convert PDF to Word format, but first the PDF image has to be OCRed (Optical Character Recognition) to convert it from an image to actual selectable/searchable text. Only then it can be converted to Word or other formats. There are a number of apps in the App Store that do both, at a wide range in cost, and equally wide range of success. Personally I use PDF Pen Pro (~$99) but PDF Pen (~$49) should do what you want unless you want to create PDF forms. You should look at the other App store offerings, read the ratings, consult your bank balance
, and make your own choice.