The font size when reading pdf in MoonReader is really small. Is there any way to increase it?
No - pdf's are statically sized documents. One of the benefits of different formats such as EPUB and amazon's MOBI files are that they are reflowable and built with html and css. This is what allows you to adjust font size in relation to the container. The text then reflows throughout the container at the new font level.
However, you can zoom on pdf's and as long as the document isn't one big image the fonts should stay highly readable.