Adobe Acrobat Reader is the free global standard for reliably viewing, signing, and commenting on PDF documents. The free Adobe Acrobat Reader PDF app is trusted by millions of people around the world. So basically Reader is free and it allows you to work with existing PDF documents. Acrobat can do everything Reader can do but also a lot more. You pay for them either with a monthly subscription fee or a one-time flat fee.
Adobe Acrobat is a paid program that comes in two versions Acrobat Standard and Acrobat Pro.