If PDF uses vectorial fonts and document is saved not as sequence of images, changing the zoom of the PDF increases/decreases the size and they do not loose quality because the size change is vectorial.
However, if PDF is saved "as graphic" a size different from 100% will decrease quality of the document.
By the way, you cannot change font size or font type. You only decrease or increase the general zoom. You will see that lines will have the same words and page the same lines.