Hi and congrats on your new Kindle
As you've found already, PDF reading on the Kindle can be a mixed bag! You cannot change the font size - merely enlarge the whole document.
When you have time, you might want to try converting the PDF into a kindle file ie .mobi This can be hit and miss depending on the complexity of the PDF but it is worth a shot. You can use software like Calibre to do that - or look online for PDF conversion.
Enjoy your new device