Texts different weight when exported as pdf

Hi, 

When I export my project as a PDF, the text's weight seem to be altered where some texts are bold and others are light. I have attached some photos of what the original looks like within Illustrator, and what it looks like when exported and opened on the Preview app. Is there a certain setting I need to change so that it exports well? 

This is original:

This is what it looks like when exported as a PDF: